Application of Electrical Conductors of a Solar Cell

ABSTRACT

A method is disclosed for applying an electrical conductor to a solar cell, which comprises providing a flexible membrane with a pattern of groove formed on a first surface thereof, and loading the grooves with a composition comprising conductive particles. The composition is, or may be made, electrically conductive. Once the membrane is loaded, the grooved first surface of the membrane is brought into contact with a front or/and back of a solar cell. A pressure is then applied between the solar cell and the membrane(s) so that the composition loaded to the grooves adheres to the solar cell. The membrane(s) and the solar cell are separated and the composition in the groove is left on the solar cell surface. The electrically conductive particles in the composition are then sintered or otherwise fused to form a pattern of electrical conductor on the solar cell, the pattern corresponding to the pattern formed in the membrane(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part (CIP) of InternationalApplication Number PCT/IB2017/054626, filed on Jul. 28, 2017, whichclaims priority from Patent Application Number GB1613051.0 filed on Jul.28, 2016, and from Patent Application Number GB1709427.7, filed on Jun.14, 2017. This application is related to simultaneously-filed U.S.applications Ser. No. ______ titled “Application of ElectricalConductors to an Electrically Insulating Substrate” (attorney docket1824US_Landa23-002) which claims priority from PCT application No.PCT/IB2017/054629 and is a CIP application thereof, and Ser. No. ______titled “Apparatus for Application of a Conductive Pattern to aSubstrate” (attorney docket 1826US_LANDA23-003) which claims priorityfrom PCT application No. PCT/IB2017/054632 and is a CIP applicationthereof. The entire disclosures of all of the aforementionedapplications are incorporated by reference herein for all purposes as iffully set forth herein.

FIELD

The present invention relates to applying a pattern of electricalconductors to a solar cell.

BACKGROUND

Known methods of forming conductors on solar cells suffer from severaldisadvantages, amongst them the limitation that they place on lineresolution and exact placement. Often, the techniques are cumbersome,can only be implemented using batch processing and when used to applyconductors to opposite sides of a solar cell, each side has to beprocessed separately.

SUMMARY

With a view to mitigating at least some of the foregoing disadvantages,there is provided in accordance with a first aspect of the presentdisclosure a method of applying a pattern of electrical conductors to asubstrate formed by a surface of a solar cell, which method comprises:

-   -   a) providing a flexible membrane, wherein a first surface of the        membrane has a pattern of grooves formed therein, the pattern        corresponding at least partially to a desired pattern of        electrical conductors to be applied to the substrate,    -   b) loading into the grooves of the first surface of the membrane        a composition that includes, as composition components,        electrically conductive particles and an adhesive, said loading        being performed in one or more sub-steps (or filling cycles)        such that on completion of the loading step the composition        substantially fills the grooves, level with the first surface of        the membrane, and parts of the first surface between the grooves        are substantially devoid of the composition,    -   c) contacting the membrane with the substrate, with the first        surface of the membrane facing the substrate,    -   d) applying pressure to the membrane to cause the composition        loaded into the grooves in the first surface of the membrane to        adhere to the substrate,    -   e) separating the membrane from the substrate to transfer the        composition from the grooves in the first surface of the        membrane to the substrate, and    -   f) applying sufficient energy to sinter the electrically        conductive particles in order to render electrically conductive        the pattern of composition transferred to the substrate from the        grooves.

In some embodiments, the step of providing the flexible membrane may bedivided into two steps: a first step of providing a flexible membrane;and a second step of forming a pattern of grooves in a first surface ofthe membrane, the pattern corresponding at least partially to a desiredpattern of electrical conductors to be applied to the substrate. In someembodiments, the pattern of grooves is formed by advancing a continuousmembrane between a die roller and a counter die, the die roller havingprotruding rules complementary to the pattern of grooves to be formed onthe first surface of the membrane.

When forming a pattern of conductors, a liquid carrier may be added tothe components of the composition to form a wet composition having theconsistency of a liquid or a paste. The liquid carrier can consist of anorganic solvent or of an aqueous solvent. A wet composition wherein theliquid carrier comprises at least 60 wt. % of water can be termed anaqueous or water-based composition, whereas a wet composition whereinthe liquid carrier comprises at least 60 wt. % of organic solvents canbe termed a solvent-based composition.

A doctor blade may be used during the step of loading, to press thecomposition into the grooves and wipe away the composition from theparts of the substrate between the grooves. The relative flexibilityand/or hardness of the tip of the squeegee or blade contacting the firstsurface of the membrane to load the compositions applied thereon withinthe grooves may be selected and adapted to the consistency/viscosity ofthe composition being loaded, the dimensions of the grooves to befilled, the force being applied to “wipe” the composition within thepattern and like considerations readily appreciated by the skilledperson. The liquid carrier may then be driven off by application of heator vacuum to leave the dried composition coating the first surface ofthe flexible membrane or the selected regions therein.

Driving off of the liquid carrier (e.g., the liquid being removed byevaporation) tends to cause shrinkage of the composition remaining inthe grooves. It is noted that while the loading step (described as stepc) in the afore-mentioned method) may include only a single filling ofthe grooves, it may optionally comprise a plurality of sub-steps, suchas repeated filling sub-steps, with optional sub-steps of liquid removal(e.g., by drying) and cleaning there-between (e.g., by wiping). Suchsub-steps may need to be repeated until the dry compositionsubstantially fills the grooves, level with the first surface of themembrane. If the loading step is carried out by repeated cycles offilling, and/or drying, and/or cleaning, the last step would leave thespaces between the grooves on the first surface of the membranesubstantially devoid of the composition.

By substantially devoid, it is meant that residual composition in thespace in between the grooves, if any, is in an amount/distributioninsufficient or too scarce to significantly mask the surface of thesolar cell following transfer. The spaces between the grooves aresubstantially devoid of composition if 2% of less of their areacomprises trace amounts of electrically conductive particles, or lessthan 1%, or less than 0.5% or less than 0.1%. Such trace amounts ofcompositions can be detected and estimated by routine image analysis.

When the loading of the grooves is carried out in several steps, whichcan also be referred to as filling cycles, the relative proportions ofthe components of the composition may be varied between filling steps.In the first step or steps, the adhesive may only serve to bond theelectrically conductive particles to one another but in the last stepthe adhesive can be relied upon additionally to cause adhesion of thecomposition to the substrate. For this reason, the proportion ofadhesive in the composition may optionally be increased at least in thelast filling step or cycle.

Additionally or alternatively, the type of adhesive agent may bemodified in between filling steps, the adhesive agent of the lastfilling step being more potent and/or in higher amount than the adhesiveor earlier steps, “potency” relating to the ability to adhere to thesubstrate. In some embodiments, an adhesive coating may be furtherapplied over the filled grooves of the flexible membrane.

The composition with a lower amount of adhesive (and/or a less potentone, which can then be considered as a binder) and a higher amount ofelectrically conductive particles may be called the metal paste, whereasthe composition with a higher amount of adhesive (and/or a more potentone), appropriate for the adhesion of the dried composition to thesubstrate, may be called the adhesive paste. The term “paste” is notmeant to indicate any particular viscosity, or solid content, howeverless viscous compositions may understandingly require more evaporationof the carrier. The composition which may be optionally added to coverthe filled grooves of the membrane to further facilitate subsequentadhesion to a substrate may be called an adhesive coating.

Alternatively, and additionally, each filling step can be performed witha different type of wiper (e.g., squeegee or doctor blade) each adaptedto the composition being loaded within the grooves and/or to the loadingconditions.

While a properly applied doctor blade or squeegee may provide thedesired result of leaving the non-grooved portions of the membrane'sfirst surface sufficiently clean of the composition, different steps mayoptionally and/or additionally be taken to achieve such goal, including,by way of example, rubbing, wiping, brushing and the like, in order toremove the composition left on the surface between the grooves, so as tocomplete the step of loading. Such cleaning may take place betweenfilling sub-stages, or only prior to the completion of the loading step,after the last filling sub-step took place and optionally before theapplication of an adhesive coating, if applied to the membrane followingloading. The optional cleaning is preferably carried out with a cleaningdevice adapted to the composition to be cleaned and to the membrane.

Any cleaning step, if performed, should not affect the surface of themembrane in a manner that would significantly reduce or preventsubsequent contact with the substrate. It is to be noted that thecleaning sub-step can be performed either before or after the drying ofthe compositions. If a cleaning sub-step is performed after a dryingsub-step, then a cleaning liquid may, if desired, be further applied tothe cleaning device and/or to the surface to be cleaned.

For instance, cleaning can be performed by a blade wiper positioneddownstream of the filling device (e.g., upstream of a squeegee or doctorblade loading the composition within the grooves). The cleaning bladecan have the same orientation as the loading blade with respect to themembrane or can form an opposite angle. The cleaning can alternativelybe performed using a cleaning roller having a soft wiping surface. Ifthe cleaning is performed following a drying step, a cleaning liquid canbe used to facilitate the displacement of the cleaning device over thesurface of the membrane. Such a cleaning liquid, if used, can alsofacilitate the dislodgment of dry composition residues, if any in thespaces between the grooves. In the afore-provided illustrative examples,the cleaning liquid could be applied downstream of a cleaning blade orserve to impregnate the wiping surface of a cleaning roller. Thecleaning liquid is selected so as not to affect the compositions alreadyloaded and/or dried within the pattern of grooves. For instance, thecleaning liquid can be a solvent compatible with the flexible membraneand incompatible with the composition liquid carrier.

The grooves of any desired pattern can be characterized, for instance,by their cross-section profile, by their top view shape, by theirdimensions, by their distance from one another and such factorsdescribed in more detail below. In some examples, the grooves of thepattern may be substantially identical with one another, whereas inother examples the grooves of a pattern need not be identical, and anyfirst groove may differ from any second groove (e.g., in depth, width,profile of a cross section, and/or delineation). Additionally oralternatively, in some embodiments, an individual groove may also varyalong its own length. For instance, from a top view perspective, asegment of a contiguous groove may form a straight delineation andanother segment a curved one, or a first segment may have a first widthand a second segment a second width and the like variations. From across section view, an individual groove may have a first profile in afirst segment and a second profile in a second segment and/or a firstset of dimensions (e.g., any cross-sectional dimension such as area,depth and/or width) in a first segment and a second set of dimensions ina second segment of the groove. By way of non-limiting example, anindividual groove may comprise a first segment having a triangular ortrapezoidal profile of a first height/depth and a second segment havinga semi-elliptic or semi-circular profile of a different secondheight/depth. A flexible membrane harboring such non-identical grooves,where an individual groove may consist of relatively shallow trenchsegments and relatively deeper indentations which may alternate alongthe length of the groove the deeper indentations forming, followingtransfer, higher spots of conductive material, or contact pads, whichmay serve to interconnect adjacent patterns that may be formed in thedirection perpendicular to the plane of the substrate, or to connect apattern of electrically conductive lines to an external circuitry (e.g.,to a battery able to store the electrical energy harvested by the solarcell), or to connect patterns of electrically conductive lines on thesolar cell to respective patterns of electrically conductive lines onother solar cells (e.g., to form solar modules, which in turns can beinterconnected to form larger arrays constituting solar panels).

The membrane in some embodiments is sufficiently flexible at operatingtemperature (e.g., at ambient room temperature circa 23° C.) tofacilitate patterning of the flexible membrane, filling of the grooveswith any of the compositions as herein taught, drying of thecompositions, or any other required processing, release of the driedcompositions from the membrane upon contacting with the substrate,and/or separation of the membrane from the substrate. While in someembodiments, the flexible membrane may be preformed and provided as such(e.g., in rolls or sheets), the flexible membrane may alternatively becast from suitable materials (e.g. thermoplastic polymers that solidifyupon cooling or photopolymers that cure upon exposure to radiation whichcan be referred herein as plastics polymers) to form a membrane as partof the process. Formable plastics polymers that can be embossed or castare known to the skilled persons.

In alternative embodiments, the embossing of a flexible membrane togenerate the desired pattern of groves can be performed at an elevatedtemperature (e.g., above ambient room temperature). Hot or thermalembossing enables the formation of grooves in membranes made of polymershaving a relatively high softening temperature compared to the softeningtemperature of polymers that are amenable to embossing at ambienttemperature (“cold embossing”).

Flexible membranes made from such embossable or castable polymers arepreferably sufficiently non-elastic to maintain the contour of thegrooves (and patterns) to be formed thereupon and the shape of thecompositions being filled therein. On the other hand, the membranes arepreferably flexible enough to conform to the surface of the substrate,so as to permit a sufficiently intimate contact for transfer of thecomposition lines. The ability of flexible membranes to intimatelyfollow the surface topography of a rigid substrate (e.g., a deliberatetexture or a casual roughness or waviness) may additionally allow suchtransfer to be performed in the absence of an adhesive layer on themembrane and the absence of an adhesive layer on the substrate; or inthe absence of an adhesive layer on any significant part of the membraneand the absence of an adhesive layer on any significant part of thesubstrate.

Yet, in some embodiments a thin layer of adhesive may be applied on thearea of the membrane that includes the pattern to be transferred, so asto facilitate the pattern adhesion to the substrate being contactedtherewith and/or so as to assist the extraction of the pattern from themembrane. In other embodiments, and for similar reasons, a thin layer ofadhesive may be applied on the area of the substrate upon which thepattern is to be transferred from the membrane, while in yet anotherembodiment a thin layer of adhesive may be applied on both the membraneand the substrate. By “thin layer”, with respect to an adhesive thatcould be optionally applied to one or two of a flexible membrane and asubstrate, it is meant a coating having an individual or a combineddried thickness of 2 μm or less, 1.5 μm less, 1 μm or less, or 0.5 μm orless.

The ability of a flexible membrane to conform to the surface of asubstrate, also referred to as the conformability of the membrane, maydepend a) on the membrane itself (e.g., its hardness, its crystallineorganization (or lack thereof), its viscoelastic properties, etc.); b)on the process used for the contacting the membrane and the substrate(e.g., the pressure and/or the temperature being applied); or c) acombination of both (e.g., when contacting is done at a temperaturearound the softening temperature of the membrane).

A membrane having a relatively smooth surface can be desired for similarreasons of improving interfacing during the process (e.g., with asqueegee filling the grooves, with the substrate, etc.). Advantageouslythe mean roughness Rz of the first surface of the flexible membrane isof 1 μm or less, 500 nm or less, 250 nm or less, or 100 nm or less.Other desirable mechanical properties can be readily understood, so asto render the membranes compatible with the method (e.g., stretchresistant, stress resistant, heat resistant, radiation resistant, andthe like) and with the compositions used therein (e.g., chemicallyresistant, chemically inert, etc.)

Flexible membranes with low surface energy can be used in the methoddisclosed herein, membranes including thermoplastic polymers selectedfrom cyclic olefin copolymer (COC), polypropylene (PP), polyethylene(PE), and thermoplastic polyurethane (TPU) being particularly suitable.

Flexible membranes may be formed of a single layer comprising one ormore of suitable plastics polymers, in particular one or more of theafore-mentioned thermoplastic polymers. Alternatively, the flexiblemembrane can be formed of two or more distinct layers (e.g., each of oneor more of the distinct layers comprising one or more of suitableplastic polymers). For instance, by way of non-limiting example, themembrane can comprise a first support layer predominantly providingmechanical integrity and/or strength (e.g., ability to substantiallyretain its dimensions during the process) of the membrane, a secondlayer sufficiently deformable so as to permit formation of grooves and athird layer “sealing” the intermediate patternable core of the membrane,improving for instance the adhesion of the membrane to the substrateahead of transfer of the pattern and subsequent separation of themembrane or any other desirable property which may be preferred for themethod according to the present teachings (e.g., providing sufficienttoughness to maintain the pattern as desired). All such layers arepreferably flexible so as to provide the desired flexibility of theoverall membrane formed thereby. In such embodiments, where the membraneis formed from two or more layers, the layer opposite the support layerconstitutes the first surface of the membrane due to contact the surfaceof the substrate during the transfer step.

The transfer of the dried compositions from the flexible membrane to thesubstrate upon application of pressure (e.g., pressure within a range ofabout 0.1 kgF/cm² to about 50 kgF/cm², or more specifically within arange of about 7 kgF/cm² to about 10 kgF/cm²) can optionally beconducted at elevated temperatures. Such transfer temperature may dependon the flexible membrane and on the substrate to be so contacted. It mayalso depend on the compositions loaded within the grooves and howtemperature may promote their release from the flexible membrane and/ortheir adhesion to the substrate. The transfer temperature can be, forinstance, of at least 60° C., at least 80° C., at least 100° C. or atleast 120° C.; and optionally of at most 200° C., at most 180° C., atmost 160° C. or at most 140° C. In some embodiments, the pressure isapplied at a transfer temperature in the range of 130-140° C.

In some embodiments, the transfer temperatures are achieved by jointlyheating the substrate and the membrane, for instance by contacting thembetween two rollers, at least one of the cylinders being heated to atemperature allowing their rapid attaining of the desired transfertemperature. In other embodiments, the transfer temperatures areachieved by heating solely the substrate, while the membrane remains oris maintained at ambient temperature. The substrate can be heated by anysuitable method, such as by conduction (e.g., passing over a hot plate),by convection (e.g., using a hot air flow), by radiation (e.g., using anIR lamp) or by combination of such heating means.

In some embodiments, the temperature is decreased after pressure isapplied to effect proper contact and before the membrane is peeled awayto effect transfer of the content of the grooves to the substrate. Atthis intermediate stage, the membrane is said to be “substrate attached”or the substrate “membrane attached”, even though this attachment istemporary until separation takes place. The cooling can be effected byconduction (e.g., passing the substrate attached membrane over a coldsurface, serving as a heat sink), by convection (e.g., blowing airoptionally cooled towards the substrate attached membrane) or bycombination of such cooling means.

The substrate is selected and adapted to the intended use of the patterntransferred thereto, such selection being known to the skilled person.The substrate can be rigid or flexible, made-up of one or more layers ofmaterials, serve for transfer on one or more of its sides, can be doped(e.g., p-doped or n-doped) or undoped, and the like. By way ofnon-limiting example, a substrate suitable for the preparation of amonofacial or bifacial solar cell can be a rigid wafer or a flexiblefilm. The substrate may be a face of a solar cell made of inorganicmaterials, such as mono or multi-crystalline silicon (mono c-Si or multic-Si), amorphous silicon (a-Si), gallium arsenide (GaAs), poly-silicon(p-Si), and any other like substrate used in solar cells, or of organicmaterials, such as flexible polymers.

Advantageously, the present method is also highly suitable forsubstrates that are relatively fragile, and/or relatively brittle (suchas glass and ceramics), and/or relatively thin (e.g., having a thicknessof 500 μm or less, 400 μm or less, 300 μm or less, or 200 μm or less).Such substrates are particularly sensitive to manufacturing conditionsand may readily break under conditions that are acceptable for morerobust and/or thicker substrates.

It will be appreciated that the present method, relying on a flexiblemembrane for the formation of a desired pattern, can be particularlyadvantageous when the solar cell substrate is not planar. Substratesthat can be bent, folded and even wrapped around objects, as may bedesired to achieve certain applications, can be contacted in one step bya membrane comprising the desired pattern. For example, assuming anobject requiring a board to fold at an angle between a left panel and aright panel of the same board, current being able to flow from one endto the other, present technologies may require the separate preparationof each panel, their assembly and possibly even the need for anintermediate connecting panel or circuitry. In contrast, the presentmethod may permit the continuous contacting of the left panel with thecorresponding part of the pattern on the flexible membrane and of theright panel with its respective pattern, the intermediate “angle” areabetween the left and right panels (and corresponding left and rightparts of the pattern) providing for an uninterrupted circuitry bridgingthe two.

Further steps may depend on the intended use of the metal pattern.

When used in the production of solar cells, the substrate may be asemiconductor wafer. In such embodiments, the composition advantageouslymay include a glass frit, and concomitantly with, or subsequent to thecomposition being sintered by heating to render it electricallyconductive, the substrate and the composition may be fired to cause theconductors to fuse with the substrate.

In other aspects, set forth in the appended claims, the inventionprovides a membrane for the manufacture of a solar cell and solar cellsproduced by methods of the invention. Such membranes and solar cells, aswell as the solar modules and solar panels including the solar cellsprepared according to the present teachings, can be characterized byvarious features, illustrated herein in more details in connection withthe diverse embodiments of the method, as herein taught, someembodiments of which are also described immediately below.

In accordance with some embodiments, there is provided a flexiblemembrane suitable for applying a pattern of electrical conductors to asubstrate formed by a surface of a solar cell, the membrane having afirst surface that includes a pattern of grooves, the grooves beingloaded so as to be substantially level with the surface of the membranewith a composition that includes electrically conductive particles andan adhesive, the composition being adapted to become electricallyconductive upon sintering by application of energy thereto, andungrooved parts of the first surface being substantially devoid of thecomposition, the membrane being such that upon pressing the membrane andthe substrate against one another, the composition in the groovesadheres more strongly to the substrate than to the membrane, and suchthat subsequent separation of the membrane from the substrate results inthe composition remaining on the substrate in a pattern mirroring thatof the grooves.

In accordance with some embodiments, there is provided a substrateformed by a surface of a solar cell, wherein a pattern of electricalconductors is applied to the substrate by a method comprising (a)providing a flexible membrane, wherein a first surface of the membranehas a pattern of grooves formed therein, the pattern corresponding atleast partially to a desired pattern of electrical conductors to beapplied to the substrate, (b) loading into the grooves of the firstsurface of the membrane a composition that includes, as compositioncomponents, electrically conductive particles and an adhesive, saidloading being performed in one or more sub-steps (of filling cycles)such that on completion of the loading step the compositionsubstantially fills the grooves, level with the first surface of themembrane, and parts of the first surface between the grooves aresubstantially devoid of the composition, (c) contacting the membranewith the substrate, with the first surface of the membrane facing thesubstrate, (d) applying pressure to the membrane to cause thecomposition loaded into the grooves in the first surface of the membraneto adhere to the substrate, (e) separating the membrane from thesubstrate to transfer the composition from the grooves in the firstsurface of the membrane to the substrate, and (0 applying sufficientenergy to sinter the electrically conductive particles in order torender electrically conductive the pattern of composition transferred tothe substrate from the grooves.

In accordance with some embodiments, there is provided a solar cell, asolar module or a solar panel having a pattern comprising a pattern ofelectrical conductors applied on a light harvesting side thereof,wherein at least part of the electrical conductors fulfil the followingfeatures: a) the electrical conductors have a cross-section profilewherein W_(B) represents a width of a base of said profile, the basecontacting the light harvesting side of the solar cell, h represents anorthogonal distance between the base and a top point of the profile(also termed the height of the profile), and ASP represents an aspectratio between the height of the profile and the base width (ASP=h/WB),wherein (i) W_(B) is at most 50 (ii) h is at most 50 μm, and (iii) ASPis at least 0.7:1 and at most 5:1; b) for at least five electricalconductors of said part of the electrical conductors, at least ten widthmeasurements are made at spaced points along each conductor and a meanwidth value and a standard deviation is calculated for each conductor,the average of the standard deviations calculated for the at least fiveelectrical conductors is no more than 5% of the average of the meanwidth values calculated for the at least five electrical conductors(these measurements and calculations defining a value which can also bereferred to as a “width variability”), c) for at least five electricalconductors of said part of the electrical conductors, at least tenheight measurements are made at spaced points along each conductor and amean height value and a standard deviation is calculated for eachconductor, the average of the standard deviations calculated for the atleast five electrical conductors is no more than 5% of the average ofthe mean height values calculated for the at least five electricalconductors (these measurements and calculations defining a value whichcan also be referred to as a “height variability”), and d) theelectrical conductors have a normalized contact resistance of at most0.3 Ω.cm. For avoidance of a doubt, width variability and heightvariability are measured and calculated for at least five electricalconductors intended to be substantially identical by design. In someembodiments, each height measurement of the at least ten heightmeasurements is made at a spaced point lying on a same cross-section asthe spaced point where a corresponding width measurement of the at leastten width measurements is performed. In some embodiments, all theelectrical conductors constituting the grid lines (also referred toherein as fingers) of a conductive pattern of a solar cell, solar moduleor solar panel prepared according to the present teachings fulfil theafore-mentioned a) to d) features.

In some embodiments, at least part of the electrical conductors appliedon the light harvesting side of said solar cell, solar module or solarpanel further have e) for at least five electrical conductors of saidpart of the electrical conductors, at least ten measurements ofcross-section area are made at spaced points along each conductor and amean cross-section area value and a standard deviation is calculated foreach conductor, the average of the standard deviations calculated forthe at least five electrical conductors is no more than 5% of theaverage of the mean cross-section area values calculated for the atleast five electrical conductors (these measurements and calculationsdefining a value which can also be referred to as a “cross-section areavariability”). For avoidance of a doubt, cross-section area variabilityis measured and calculated for at least five electrical conductorsintended to be substantially identical by design. In some embodiments,each cross-section area measurement of the at least ten cross-sectionarea measurements is made at a spaced point where a corresponding widthmeasurement of the at least ten width measurements and a correspondingheight measurement of the at least ten height measurements areperformed. In some embodiments, all the electrical conductorsconstituting the grid lines of a conductive pattern of a solar cell,solar module or solar panel prepared according to the present teachingsfulfil the afore-mentioned a) to e) features.

In some embodiments, at least part of the electrical conductors appliedon the light harvesting side of said solar cell, solar module or solarpanel further have f) a sum of lengths of any segments in thecross-section profile having slope less than 1 (the segment beinginclined to the plane of the substrate at an angle of less than)45° ofat most 15 μm, at most 12 μm, or at most 10 μm. In some embodiments, allthe electrical conductors constituting the grid lines of a conductivepattern of a solar cell, solar module or solar panel prepared accordingto the present teachings fulfil the afore-mentioned a) to f) features.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementation of the present disclosure will now be described further,by way of example, with reference to the accompanying drawings, inwhich:

FIGS. 1A to 1D are sections showing simplified process steps in forminga pattern of lines of a composition containing electrically conductiveparticles on a membrane and then transferring the pattern to asubstrate;

FIG. 1E depicts a simplified diagram of a process to implement theprocess of FIGS. 1A to 1D;

FIG. 2A is a perspective view of the process step shown in FIG. 1Aproviding an example of how the line pattern may be designed to form acontinuous membrane having discrete spaced patterns, each intended forapplication to a separate respective substrate, and each patterndefining a set of parallel conductors connected to one another bytransverse bus bars;

FIG. 2B and 2C are respectively enlarged views of the circles designatedB and C in FIG. 2A showing the grooves in cross-section;

FIG. 3A is a section through a membrane showing how a single compositionmay be applied for forming the lines of composition within grooves inthe membrane;

FIG. 3B is a section through a membrane showing how two distinctcompositions may be consecutively applied in the groove for forming thelines of composition in the membrane;

FIG. 3C is a section through a membrane showing how a release layer mayfirst be applied to assist in separation of the composition lines fromthe membrane;

FIG. 3D is a section through a membrane showing how an adhesive coatingmay be applied to cover the entire surface of the membrane to assist inthe transfer of the lines of composition from the membrane to thesubstrate;

FIGS. 4A to 4C show how an electrode, such as the back electrode of asolar cell by way of example, may be formed and applied to a largesurface;

FIG. 5A schematically illustrates an exemplary metallic pattern asgenerated by the present method;

FIG. 5B schematically illustrates an exemplary cross-section through aprotruding die rule, which is similar in shape to a cross-sectionthrough a transferred contact line before and after sintering andcomparable to a negative image of a cross-section through a groove;

FIG. 5C is a perspective micrograph taken by confocal laser scanningmicroscopy of a groove of a flexible membrane, the groove being filledwith a composition including electrically conductive particles;

FIG. 5D is a perspective micrograph taken by confocal laser scanningmicroscopy of a contact line transferred from a groove of a flexiblemembrane to a substrate; and

FIG. 6 schematically illustrates an exemplary cross-section of anapparatus for filling a plurality of groove sets with respectivecompositions.

DETAILED DESCRIPTION

The ensuing description, together with the figures, makes apparent to aperson having ordinary skill in the pertinent art how the teachings ofthe disclosure may be practiced, by way of non-limiting examples. Thefigures are for the purpose of illustrative discussion and no attempt ismade to show structural details of an embodiment in more detail than isnecessary for a fundamental understanding of the disclosure. For thesake of clarity and simplicity, some objects depicted in the figures maynot be drawn to scale.

For brevity and clarity, the description is generally directed toforming a pattern as shown in FIG. 2A (enlarged views of which are shownin FIGS. 2B and 2C), by way of illustrative example only. The skilled inthe art would readily understand that the pattern formed by grooves, andensuing corresponding conductors pattern may take any form, and is amatter of technical choice dictated by the matter at hand.

Flexible Membrane Patterning

In FIG. 1A, there is shown a membrane 100, made of a plastics material,that is passed through a nip between a cylindrical pressure roller 104(which may also be referred to as a counter die) and a die roller 102.The die roller 102 can be formed of a cylinder from the smooth surfaceof which there project rules 106 and 108. The manner in which the dieroller 102 is formed is not of fundamental importance. One method of itsmanufacture may be by etching a smooth cylinder and an alternative wouldbe to mount an embossing shim (typically made of nickel or chrome) withprotruding rules 106 and 108 around a cylinder. While not shown in thefigure, the flexible membrane 100 can be formed of one or more layers ofdistinct flexible materials.

Rules 106 are parallel to the circumference of die roller 102 and arealigned with each other along the axis of the die roller so as to formupon contacting the facing surface of the membrane, grooves parallel tothe direction of movement of the membrane. Only one such rule can beseen in the schematic cross-section of FIG. 1A. Rules 106 need notfollow the entire circumference of the die roller, their length beingadjusted to the desired length of the longitudinal lines in the groovespattern. Rules 108 are parallel to the axis of die roller 102 and canform upon contacting the facing surface of the membrane 100, groovestransverse to the direction of movement of the membrane, and optionallyorthogonal to the longitudinal grooves formed by rules 106. Rules 108need not extend along the entire length of the die roller, or width ofthe membrane, their length being adjusted to any desired number oflongitudinal lines to be traversed in the grooves pattern. The lengthsof rules 106 and 108 may also take into account the dimension of theintended substrate and margins that may be required to surround anyparticular pattern. Though rules 106 and 108 are illustrated as straightlines, it is readily understood that a die roller 102 may carry rulesforming any other desired shape.

The edges or profile of the rules can have any desired shape, typicallyregular, allowing for the transfer of the composition lines from themembrane to a substrate, and therefore have desirably a tapering form.Such a profile is schematically illustrated by a semi-ellipse in FIGS.1A-1D. The edges of rules 106 can be, by way of example, trapezoidal,that is to say that the rules may have upwardly tapering sides and aflat top. The width of the base of the trapeze is herein denoted WB, thewidth of the flat top WT, and the height between the two, h, as will bediscussed in more details with reference to FIG. 5A. Rules 108 may haveedges similarly shaped to those of rules 106, but this need notnecessarily be the case. For example, rules 108 may be formed from anassembly of protrusions rather than from a single shape, as previouslyexemplified by a trapeze for rules 106. Rules 108 can for instance beformed by a strip of mesh material. For the manufacturing of solarcells, wherein rules 108 may serve for the later formation of bus bars,the width of the mesh stripe can be larger than the width of the base ofrules 106. Generally, the number of rules 108 is smaller than the numberof rules 106 for the metal patterns applied in the depicted example ofelectrodes for a solar cell.

Though in the following, the terminology assigned to trapezoidal crosssections shall be used, rules may have any different tapering shapesatisfying similar ranges. The top edge of a rule may come to a point ifthe rule cross section is a triangle, a semi-circle or a semi-ellipse,and the like. The width of the base of the rules may depend on itsfunction. Widths of up to a few millimetres can be suitable for busbars, and even wider rules may be suitable for solar cell back sideelectrodes, if such are prepared using a grooved membrane. Forlongitudinal lines, base widths of 50 micrometers (um) or less arepreferred, a W_(B) in the range of 10-40 μm, or 10-30 μm, or 10-20 μm,or even 5-20 μm, being particularly desired. The width of the top of therule can also depend on function, and be approximately commensurate withthe base width, though typically smaller. For longitudinal lines, topwidth of 40 μm or less are preferred, a w_(T) in the range of 5-25 μm,or 5-15 μm, or 10-20 μm, being particularly desired. The height of therules h generally does not exceed 50 μm, a h in the range of 25-40 μm,or 25-30 μm, or 15-25 μm, being particularly desired. It can beappreciated that the aspect ratio (ASP) between the height h of a rule106 and its base width W_(B) can be within a range of about 5:1 to about1:5, of about 5:1 to about 0.7:1, of about 3:1 to about 1:1, of about2:1 to about 1:2, of about 1.75:1 to about 1:1.75, or of about 1.5:1 toabout 1:1.5; optionally the ASP of about 2:1, about 1.5:1 or at least1:1, being preferred.

Additionally, the pattern can be characterized by the distance d betweenthe facing edges of adjacent grooves. Typically, such distance is noless than 100 μm. In some embodiments the distance between parallelsegments of adjacent grooves d is at least 150 μm, at least 200 μm or atleast 300 μm. For longitudinal lines, the maximal distance between twogrooves may depend on the intended use and/or the desired efficiency.For instance, for the preparation of solar cells d is at most 2,000 μm,at most 1,500 μm, at most 1,000 μm, or at most 500 μm. It is to be notedthat the dimension of the grooves and of the lines resulting from anyparticular rules, are not necessarily identical to the dimension of theoriginal rule. Such variations may depend on the membrane being used andon other variables of the process. Advantageously, such variations donot exceed 25% of the original dimensions.

Yet, the present method allows transferring to the substrate metal lineshaving a relatively high aspect ratio, generally supporting lowerresistance. Taking for illustration, a metal trace having a width of 25μm, clearly a line having a height of 50 μm, namely an ASP of 2:1, wouldhave lower resistance than a line having a height of only 5 μm, namelyan ASP of 1:5, all other parameters (e.g., metal line composition) beingsimilar.

The die roller 102, though typically significantly more intricate and/orwith rules having smaller dimensions, may be formed in the same way asused in producing dies for creasing or cutting cardboard. As themembrane 100, passes through the nip between the pressure roller 104 andthe die roller 102, grooves or indentations 110 are formed in the uppersurface of the membrane. The nip between the die roller and the counterdie can also be referred to as the patterning nip. The rules 106 and 108are designed to form a groove pattern in the membrane that correspondsto the pattern of the electrical conductors to be applied to asubstrate, which may be an insulator or a semiconductor. Though in thepresent figure, and as shown more clearly in FIG. 2A, two sets of rulesare illustrated for the formation of a depicted groove pattern in themembrane, this is not limiting, as different applications may requireonly one set or more than two sets of rules, as relevant to the desiredpatterns.

The patterning element is illustrated as a rotating die roller 102forming a patterning nip with a respective counter die (e.g., cylinder104), the pattern being formed as the membrane moves through the nip(e.g., the membrane and the patterning element being in relative motionduring the patterning). However, alternatively shaped assemblies ofrules may be suitable to form grooves as above described. The formationof grooves in the membrane can be, for instance, achieved by plates. Ifthe process is carried by passing the membrane through fixed stations,such a groove forming station and a groove filling station to load thecompositions, lengths of slack membrane between the stations can permitthe process to be performed continuously. As sub-steps of the loadingstep may be repeated, there might be more than one filling stationwherein the compositions may be applied and dried, each station can beoperated under different conditions, in particular when the compositionsbeing loaded within the grooves differ from one repeat to another.

In addition to embossing the pattern of grooves within a preformedflexible membrane of one or more layers, the membrane can be cast from arelatively viscous yet fluid state material, the grooves being formedprior to the hardening of the film. The pre-hardened material can bereferred to as “pre-membrane” material. For instance, the membrane canbe formed by extrusion of pre-membrane material through a nozzle havingon at least part of its contour protrusions able to yield thecorresponding grooves in the film as it hardens. Similarly to theprocess described in FIG. 1, a fluid of pre-membrane material can becast through a smooth slot and the grooves formed by a rotating dieroller or by plates carrying the desired pattern of rules. Depending onthe materials being used, the hardening of the relatively fluidpre-membrane material into a flexible membrane with a pattern of groovesaccording to the present teachings can be achieved by cooling the filmand/or by curing it. Such processes of cold/hot embossing andUV-embossing are known and need not be further detailed herein.

The pre-membrane material can for instance be a UV-curable material,which can be cast, by way of example between a rotating die roller and atransparent counter surface (e.g., pressure roller) allowing to UV-curethe film as it passes at the nip where the grooves or indentations arebeing formed. The pre-membrane polymer or blend thereof can be cast as asingle self-supporting layer or can be cast to form a patternable layer(e.g., made of CPP) upon a relatively less deformable support layer(e.g., a PET film).

Optionally, the grooves may be formed by a directed laser beam. In suchembodiments a laser beam is directed at desired locations of themembrane, to ablate portions of the membrane, or to heat it causingshrinkage. Furthermore, the grooves may be formed by one or more punchesapplying pressure to the membrane.

FIG. 1E is a simplified flow diagram depicting process steps in applyinga pattern of electrical conductors to a substrate which may in turn forma solar cell. The process begins by providing a suitable flexiblemembrane 305 and forming a pattern of grooves 310 in a first surface ofthe membrane, the pattern corresponding at least partially to thedesired pattern of electrical conductors to be applied to the substrate.The grooves are then loaded 315 with a composition that includeselectrically conductive particles and an adhesive. The loading step,which includes filling the grooves with the desired composition thenoptionally drying it and/or cleaning the space in-between the grooves,can be repeated until the dried composition essentially levels with thesurface of the membrane. Alternatively, a subset of the grooves(partially relating to the desired pattern to be applied to thesubstrate) may be formed and loaded, and the forming and loading maythen be repeated for one or more other subsets of grooves (alsopartially relating to the desired pattern). After the loading step hasbeen completed for all the grooves, the membrane is contacted 320 withthe substrate and pressure is applied 325 between the membrane and thesubstrate, to cause the composition loaded into the grooves in the firstsurface of the membrane to adhere to the substrate. The membrane is thenseparated 330 from the substrate to transfer at least some, andpreferably substantially all, of the composition from the grooves in thefirst surface of the membrane to the substrate, and sufficient heat orother form of energy is applied to sinter or otherwise fuse 335 theelectrically conductive particles in order to render electricallyconductive the pattern of composition transferred to the substrate fromthe grooves.

The loading step, which includes filling the grooves with the desiredcomposition(s), may also include substantially drying or otherwisestabilizing the composition(s). Such steps can be repeated until thesufficiently dried composition is essentially level with the surface ofthe membrane. The loading step may also include one or more cleaningsteps, which may be performed simultaneously with, or separately from,the one or more filling steps by the likes of a doctor blade, asqueegee, a wiper, and the like. Alternatively or additionally, thefirst surface of the membrane may be wiped brushed, or otherwisecleaned.

From the perspective view of FIG. 2A, it may be seen that the membranemay be a continuous membrane on which the same groove pattern is appliedrepeatedly. The pattern may comprise parallel lines extending parallelto the length of the membrane, these longitudinal lines being formed bycircumferential rules 106 on the die roller 102, as well as transversebus bars, which are formed by rules 108 extending axially on the surfaceof the roller 102. An enlarged cross-section view of a few exemplaryrules 106 is shown in FIG. 2B. Helical rules (not shown) on the dieroller 102 may be used to form diagonally extending grooves, likewiserelatively curved connecting rules may serve to join the linear ones sofar described, thereby enabling any desired pattern of conductors to beformed. Though for simplicity of illustration, the rules 106 or 108 areshown has having same length among their group, this need not be thecase (see 520 and 530 in FIG. 5A by way of example).

The Flexible Membrane

As explained, it is required of the membrane that it should not have theresilience to return to its original shape after passing through thenip, beneath plates, or after the grooves are being formed in anydesired manner. The membrane may suitably be formed of one or moreplastics polymers, in particular thermoplastic polymer(s) selected fromthe group comprising cyclic olefin copolymer (COC), ethylene-vinylacetate (EVA), polyamide (PA), polycarbonate (PC), polyethylene (PE),polyethylene terephthalate (PET), polypropylene (PP), polyurethane(TPU), polyvinyl chloride (PVC), and substituted versions thereof. Butit may alternatively be formed of different polymers, or of non-plasticsmaterials, such as photopolymers, that can be hardened after grooveshave been pressed into any of them. It is desirable for the membrane tobe sufficiently flexible to permit it to be coiled. It is also importantthat the membrane be sufficiently flexible to be peeled away (or inother words stripped off) from a contacting surface (e.g., of asubstrate).

It should be stressed that the above described method of forming amembrane having a groove pattern is not fundamental to the invention.Any membrane formed by extrusion, stamping or machining may be used, solong as it is pliable, and preferably flexible, and is capable ofmaintaining accurate grooves of the desired depth, width and crosssection (e.g., a trapezoidal or triangular cross section), the groovesbeing shaped and spaced as desired within the pattern.

The membrane can be supplied in sheets or as a continuous web. Thedimensions of the membrane are not limiting, but need preferably becommensurate with the patterning element and the intended substrate. Forinstance, the width of a membrane may approximately correspond to thelength of die roller 102 in its axial direction, while the length of themembrane, if provided as individual sheets, the length being parallel tothe direction of movement, is typically a low multiple of the rollercircumference. The thickness of the membrane exceeds the height of therules (i.e., the depth of the grooves) and is generally in the range of20-100 μm. If the membrane if formed by more than one layer of polymers,its thickness can be in the range of 20-150 μm, each layer beingindividually in the range of 0.5-100 μm. For instance, a support layer(e.g., made of PET) can have a thickness of 8-75 μm, apatternable/deformable layer (e.g., made of CPP) can have a thickness of10-75 μm, and a protective sealing layer optionally furthering futureproper contact with the substrate (e.g., made of ethylene vinyl acetatecopolymer resins, such as commercialized as Elvax®) can have a thicknessof 0.5-5 μm.

It is noted that patterns generated by one cycle of the die roller maysuffice to the preparation of a plurality of end products, even if thelamination contact of the membrane to the desired substrate is performedin parallel and/or in series on an area corresponding to a number ofsuch end-products.

If desired, the surface properties of the flexible membrane may bemodified (e.g., by physical or chemical treatment) to improve thesuitability of the membrane for application of any following material.For instance, if the flexible membrane is made of a hydrophobic polymerand the compositions to be subsequently applied may not sufficiently wetit or otherwise interact therewith, then a modification of the polymersurface, or part of it such as a selective modification of the grooveswall surface, may be performed. The surface can be treated (e.g., coronatreatment or chemical treatment with wetting improving agents) in orderto improve the wettability of the flexible membrane or of its grooves,so that it becomes relatively more hydrophilic. The compositions mayalso include agents improving their wetting of the membrane. Themembrane, or part thereof, and the interfacing compositions aresufficiently wettable or wetting, accordingly, if their contact isintimate enough to prevent undesired entrapment of air bubbles of a sizeand/or in an amount and/or at a density which may weaken the resultingpattern. Preferably, optimal wetting of the flexible membrane by anycomposition interfacing therewith result in a subsequently dried layerof the composition being substantially continuous and/or devoid of airbubbles.

Regardless of the method of forming the flexible membrane and patterningit, or of any facilitating modification applied thereto, the membrane istypically characterized by its ability to conform the surface of asubstrate to be thereafter contacted. While the intimate contact of themembrane with the substrate surface may bring about the deformation ofthe membrane surface in accordance with any texture, waviness orroughness of the substrate, such “replication” of the substrate surfacedoes not negatively affect the transferability of the pattern from themembrane. On the contrary, the high conformability of the membrane tothe surface of the substrate is believed to increase the contact areabetween the two, facilitating transfer there-between. It should be notedthat, as opposed to conventional methods, an irreversible alteration ofthe membrane, if any, during transfer of a composition pattern may be ofno consequence, in embodiments where the flexible membrane is disposableand can therefore be thrown away following transfer and peeling. Itshould be further noted that although deformation of the flexiblemembrane may mirror the topography of the surface of the substrate,(e.g., in the sub-millimeter or sub-micron scale), the variations intopography on the surface of the substrate may be smoothed out to acertain extent in the deformed membrane. For example, peaks on thesurface of the substrate may be halved in magnitude in the deformedmembrane.

Compositions for the Patterns of Conductive Lines

After the groove pattern has been formed in the membrane 100, the nextstep in the process is to fill the grooves with a composition thatcontains electrically conductive particles, for instance silver, and anadhesive, such as a hot melt polyamide (if transfer is performed atelevated temperature). This loading step is shown in FIG. 1B where themembrane 100 passes between a pressure roller 132 and a blade or scraper130 that squeezes the composition 120 into the grooves 110 to fillgrooves in the manner shown to the right of the blade 130 in FIG. 1B, afilled groove being shown as 140. An enlarged cross-section view ofexemplary grooves 110 before their loading is shown in FIG. 2C.

In the following, the proportion between various compounds or agentsforming a composition can be provided in weight per weight (w/w) orvolume per volume (v/v) ratio, or percentage of the composition, partthereof, or even with respect to a single other constituent, whichpercentage can be denoted wt. % and vol %, respectively.

The particles (whether of metal or of adhesive, such as glass frits) mayhave any shape, for instance form regular or irregular sphericalbeads/flakes/rods and the like, preferably the individual particles ofmetals are devoid of cavities that may prevent proper sintering orfiring at a later stage. The maximum dimension of the particles shouldbe smaller than the groove minimal size in any of its axis/dimension.(e.g., the particles are in the order of a few microns, commonly notexceeding 10-20 μm, and often significantly smaller, such as in thenanometer scale). Too large particles may not pack satisfactorily withinthe groove, depending on their shape, such deficient packing reducing orpreventing the formation of adequately conductive lines. Though smallerparticles are easier to pack in the groove, excessively small particlesmay not be beneficial. First, they may undergo uncontrolled sintering attemperatures relatively lower than the sintering temperature of largerparticles (e.g., at transfer temperature of about 135° C.). Suchpremature sintering of particles that are too small may subsequentlyaffect transfer of the paste (hampering flow) and its interfacing withthe substrate. Second, the increased surface area of numerous smallparticles may require the presence of additional adhesive, which may inturn affect the rheology of the composition and the workability of thepaste. Furthermore, the increased presence of adhesive may interferewith sintering (affecting prospective conductivity). Hence particleshaving a maximum dimension of at least 200 nm; and optionally at most 10μm, are preferred. In some embodiments, the electrically conductiveparticles have a maximum dimension in the range of 0.5-5 μm or 1-3 μm.Populations of particles heterogeneous in size, though not essential forcertain shapes, may improve the packing within the grooves, resulting inpacking having less inter-particular voids. Such packing facilitatessintering and if needed firing, and can improve the conductivity of thefinished sintered/fired line.

Information about particle size is generally provided by the suppliers,and may be determined by routine experimentation using, Dynamic LightScattering (DLS) techniques by way of example, where the particles areapproximated to spheres of equivalent scattering response and the sizeexpressed as hydrodynamic diameter. Dimensions of particles may also beestimated by microscopic methods and analysis of images captured byscanning electron microscope (SEM), transmission electron microscope(TEM), focused ion beam (FIB), and/or by confocal laser scanningmicroscopy techniques. Such methods are known to the skilled persons andneed not be further detailed. As particles typically have differentsizes in different directions, unless perfect spheres, the longestdimension in the largest plane projecting from the particle isconsidered for simplicity. When the particles are globular or nearspherical, the “longest dimension” is approximately their diameter whichcan be estimated by DLS methodology. In such a case, the hydrodynamicdiameter of 90% of the population of the particles, but more typicallyof 50% of the population, can serve to assess the size of the particles.In other words, and depending on shape, the particles can becharacterized by their longest length L, by their thickness, by theirhydrodynamic diameters at D_(V)90, D_(V)50, D_(N)90 or D_(N)50.

As used herein, the term “electrically conductive particles” encompassesparticles made of any conductive material, including metals, metaloxides, metal salts, organo metals, alloys and conductive polymers, aswell as any electrochemically compatible combinations of the foregoing(e.g., a mixture of two metals, aluminum and silver). Byelectrochemically compatible, it is meant that any conductive materialof any layer is chemically inert with respect to any other conductivematerial of the same layer, or of other layers when the loading of thegrooves is done by repeated filling sub-steps. In particular none of thematerials are deleterious to the intended effect, more specifically notaffecting the electrical conductance and/or conductivity of the ultimateconductive lines that can be obtained by the present method, nor theirability to properly attach to the substrate in due time.

Metals can be selected from the group comprising aluminum, copper, gold,silver, tin, nickel, platinum, zinc; and alloys can be selected from thegroup comprising bronze, brass and palladium/silver. Organo-metals canbe selected from the group comprising copper(II) formate (C₂H₂CuO₄),copper(II) hexanoate (C₁₂H₂₂CuO₄), copper(I) mesitylene (C₉H₁₁Cu),vinyltrimethylsilane Cu(I) hexafluoro-acetylacetonate, silverneodecanoate (C₁₀H₁₉AgO₂), precursors, hydrates and/or salts thereof.

In addition to electrically conductive particles, the composition 120includes an adhesive. Different types or amounts of adhesives may beused depending on the elected method step and the ultimate substrate.Broadly, the adhesive may be one or more of a) an organic bindersufficient to maintain the cohesivity of the electrically conductiveparticles but insufficient to provide enough adhesion to the substrate(a “poor adhesive”), b) an organic adhesive additionally adequate toprovide the desired adhesion to the substrates (a “potent adhesive”),and c) glass frits, which may be deemed an inorganic adhesive, when themethod is used at elevated temperatures into glass and/orglass-receptive substrate, such as for the preparation of solar cells.In the event a first composition lacks a potent organic adhesive of type(b), then a second composition enriched with a suitable adhesive isconsequentially used, as will be described in more details withreference to FIG. 3. Though classified as adhesive, being a fundamentalactivity of the inorganic glass frits, such compounds may fulfilladditional roles in the manufacturing of solar cells, as known to theskilled persons.

Adhesive compounds (e.g., organic adhesives) can be, for instance,pressure sensitive adhesives, if transfer is performed only underpressure, or heat sensitive adhesives (e.g., hot melt adhesives), iftransfer conditions further include elevated temperatures. The hot meltadhesives can be polymers having a softening point in the range ofrelevance to transfer, for instance between 60° C. and 180° C. Asoftening temperature in such range can optionally be achieved by mixingan adhesive agent having a relatively higher softening point with aplasticizer able to reduce such phase transition temperature. Softeningtemperatures of polymers are provided by their suppliers, but can beassessed by routine experimentation according to methods known to theskilled persons, for instance by using Differential Scanning calorimetry(DSC).

The adhesive is compatible with other components of the composition(e.g., the electrically conductive particles, the glass frits, whenpresent, and the carrier) and the process conditions, for instance,providing for a suitable flowability of the composition within thegrooves and/or excess removal from membrane surface, being non-brittleto maintain pattern integrity or being sufficiently “heat resistant”till sintering of electrically conductive particles to remain in anamount maintaining adequate shape. The adhesive should preferably have arelatively low adhesion to the flexible membrane (e.g., only sufficientfor the composition, wet or dry, to remain within the grooves duringmanufacturing and handling, but to be releasable from the membrane whenit is peeled off from the substrate after lamination). On the otherhand, the adhesive should preferably have a relatively high adhesion tothe receiving substrate (e.g., allowing for the transfer of the metalpattern to the substrate). Advantageously, adhesives having a low ashcontent are expected to facilitate metal sintering, improvingconductivity (optionally after firing). Adhesives having an open timesufficiently long to allow proper transfer of metal pattern to thesubstrate, following its optional heating to an elevated transfertemperature, while being sufficiently short to maintain the desiredshape (structural integrity of the pattern lines) following transfer,are considered suitable. It is to be noted that open times of less thana few seconds, one second, hundreds or even tens of a millisecond arepreferred.

Exemplary organic adhesives can be polyamides, including, for instance,commercially available hot melt polyamide adhesives Uni-Rez® 147,Uni-Rez® 2620 and Uni-Rez® 2720 (formerly of Arizona Chemical, now ofKraton Corporation, USA), Macromelt® 6211, Macromelt® 6238, Macromelt®6239, and Macromelt® 6264 (Henkel, Germany), Versamid® 744 and Versamid®754 (Gabriel Performance Products, USA); terpene phenolic resins, suchas Sylvaprint® 3523 and Sylvaprint® 7002 (Arizona Chemical, USA);hydrogenated Rosin, such as Foral™ AX-E (Eastman Chemical Company, USA);ethylene-vinyl acetate (EVA) copolymers, including, for instance,commercially available Elvax® 40W of E.I. du Pont de Nemours andCompany, Inc.; or ethyl cellulose polymers, including, for instance,commercially available Ethocel™ std 4, 7, 10 or 20 of The Dow ChemicalCompany.

As mentioned, adhesives may, in some embodiments, be used to form thincoating layers over the entire surface of the membrane and/or thesubstrate, or over a region of either one or both of the aforesaidsurfaces so that transfer and/or adhesion may be facilitated. Similaradhesive materials may be used for such purposes, alone or incombination. Thin coating layers not exceeding in total 2 μm when dry,such as being 1 μm thick or less when dry, or such as being 0.5 μm thickor less when dry, can be applied by any suitable method. Coating methodsinclude, by way of non-limiting example, spray coating, rod coating,flexographic printing, and screen printing.

Glass frits suitable for solar cells are generally made of the followingglasses:

-   i) Lead oxide (PbO) based glasses, usually lead borosilicate glass:    PbO—SiO₂—B₂O₃; ii) Bismuth oxide (Bi₂O₃) based glasses, usually    bismuth borosilicate glass: Bi₂O₃—SiO₂—B₂O₃; and iii) Tellurium    oxide (Tl₂O) based glasses.

Each of the above glasses may also contain one or more of the following:aluminum oxide (Al₂O₃), bismuth oxide (Bi₂O₃), boron oxide (B₂O₃), leadoxide (PbO), molybdenum oxide (MoO₂), silicon oxide (SiO₂), telluriumoxide (Tl₂O), tungsten oxide (WO₃), and zinc oxide (ZnO).

Glass frits, when present, may etch the outer surface of the substrateupon firing. Solar cells (or combinations thereof forming solar modulesor solar panels) prepared by lamination of membranes and transfer ofpatterns according to the present teachings may therefore be identifiedby the presence of fire-through contact etching underneath the firedlines.

A suitable amount of electrically conductive particles (including glassfrits when present) is 30-95 vol % with respect to all solids (e.g.,electrically conductive particles, adhesives and glass frits, ifpresent), more preferably 65-90 vol % for the metal paste and morepreferably 50-70% vol % from all solids for the adhesive paste. Theamount of glass frits, when present, is generally 0.5-15 vol % of theelectrically conductive particles, preferably in the range of 1-5 vol %.

A suitable amount of adhesive: 5-70 vol % with respect to all solids(e.g., electrically conductive particles and adhesives, including glassfrits, if present), preferably within 30-50 vol % of solids for theadhesive paste, and preferably within 10-35 vol % of solids for themetal paste.

Optional plasticizers can be phthalates, phosphates, glycerides, andesters of higher fatty acids and amides. For example, when needed, aplasticizer can be one or more of the group comprising dibutyl sebacate,butyl stearate, glycol esters of coconut oil fatty acids, butylricinoleate, dibutyl phthalate, castor oil, butyl stearate, diphenylphthalate, dicyclohexyl phthalate, and dioctyl phthalate. If present, aplasticizer can be found at 5-30 wt. % with respect to the adhesive.

The electrically conductive particles and adhesives can be mixed in aliquid carrier, to form the composition 120 that is used to fill thegrooves 110. Preferably, the amount of carrier should be sufficientlylow to shorten the time needed to eliminate it when drying thecomposition. On the other hand, the amount of carrier may need to besufficient to provide suitable flowability to the composition, allowingthe composition to fill the grooves relatively rapidly and excessthereof to be removed relatively easily from the surface of themembrane. Typically, the liquid carrier is present in the composition inthe range of 30-80 vol % of the total paste.

The liquid carrier may be aqueous, organic, or consist of mixturesthereof. Organic solvents are preferably volatile and can, for example,be selected from the group comprising linear or branched C1-C7 alcoholsand C1-C7 alkyl acetates, any such solvent preferably being of highpurity of 95% or more, typically above 98%. Such alcohols can be one ormore of methanol, ethanol, n-propanol, isopropanol, n-butanol,isobutanol, pentanol, hexanol and heptanol. Such alkyl acetates can beone or more of methyl acetate, ethyl acetate, n-propyl acetate,isopropyl acetate, n-butyl acetate, isobutyl acetate, pentyl acetate,hexyl acetate and heptyl acetate.

As explained, the composition 120 may be applied in consecutive steps,the elimination of the liquid carrier in between such steps resulting inrelative shrinkage of the dried composition as compared to the wetcomposition.

It is to be understood that intermediate drying of the compositionduring first fillings providing for a partial filling of the groovesneed not necessarily be as extensive as the drying performed subsequentto the last filling step. As used herein, a “dried” or “substantiallydried” composition may retain residual amount of liquid carrier, as longas such presence does not hamper the structural integrity of thepatterned conductor lines so dried or any other aspect of the process. Acomposition comprising less the 5 vol. %, and preferably less than 2vol. % or even less than 1 vol. % can be considered “dried”.

Once the dry composition substantially fills the grooves (see, 140 inFIG. 1B or any of 140 a-d in FIG. 3) and levels with the first surfaceof the membrane, the flexible membrane bearing metal pattern is readyfor further processing, which however need not be performed by the sameentity, nor in temporal proximity to the filling step. In such case, thesteps preceding and following the contacting of the first surface of themembrane with the solar cell substrate can be temporally separated.

For instance, once a length of membrane has been made by itsmanufacturer, it may be packaged and sent to an end user who will usethe membrane to form conductors on solar cell. The membrane may bepackaged in coils. To prevent the composition from adhering to theopposite side of the membrane when it is coiled, the second surface ofthe membrane may have “non-stick” properties with respect to the firstsurface of the membrane. Alternatively, a protective sheet, preferablyhaving a release surface (e.g., hydrophobic), may be applied to themembrane to be peeled away from it prior to the next step in theprocess.

FIG. 1C shows the next step (which can be performed by the same or adifferent entity) in which the membrane 100 is brought into contact withthe substrate 150 and the two are passed through a nip 156 between twopressure rollers 152, 154 to cause the composition 140 within thegrooves of the membrane to adhere to the surface of the substrate 150.At least one of pressure rollers 152 and 154 can be heated to atemperature typically in the range of 60-200° C. to further facilitatethe transfer. In some embodiments, the substrate and/or the membrane mayadditionally or alternatively be preheated before reaching nip 156. Forexample, the substrate may be heated to about 150° C. upstream ofpressure rollers 152 and 154, and one or both of pressure rollers 152and 154 may be heated to about 60° C.

In such a case, wherein the substrate, the membrane and/or the pressurerollers can be heated, the transferring composition may also be called amolten paste. This step may also be referred to as the lamination stage.

It should be noted that for transferring a pattern by lamination of aflexible membrane on a substrate, pressure rollers 152 and 154 typicallyhave a relatively low hardness, as compared to traditional printingmethods (such as gravure or intaglio printing, where the image-bearingplate or image-bearing cylinder at an impression nip, are typicallyrelatively harder).

In some embodiments, pressure roller 152 and/or 154 is coated with amaterial (e.g., a polymeric compound or blend) so as to provide on itsouter surface a hardness of no more than 70 Shore A, no more than 60Shore A, no more than 50 Shore A, or no more than 40 Shore A. In someembodiments, pressure rollers 152 and 154, if coated, have an outersurface hardness of at least 10 Shore 00, at least 30 Shore 00, at least50 Shore 00 (which approximately correspond to 10 Shore A), at least 20Shore A, or at least 30 Shore A.

Last, in FIG. 1D, a portion of the membrane 100 of which the compositionwas already brought into contact with the substrate 150 is pulled awayfrom the substrate 150 in the general direction of the arrow 160 whilethe membrane 100 continues to be held against the substrate 150 by aroller 170 at the point of separation, so as to peel the membrane 100 ata known region away from the substrate leaving lines 140′ of thecomposition adhering to the substrate 150.

It is to be noted that if the membrane is not immediately peeled awayfrom the surface following transfer, the prolonged contact between thepattern included in the membrane and the substrate may facilitateadhesion and/or extractability of the pattern.

At the transfer nip 156, the membrane passes between two pressurerollers 152, 154, and at roller 170, the membrane is separated from thesubstrate. During the time that it takes for the membrane to travel fromtransfer nip 156, to roller 170, the membrane and its underlyingsubstrate may spontaneously cool down to a predetermined temperature(e.g., ambient temperature or any temperature below the softening pointtemperature of adhesives and below the softening temperature of themembrane). For example, cooling by air may take about half a minute, tocool down the membrane and its underlying substrate by about 88% of thedifference between the initial temperature and ambient temperature,e.g., to cool down from about 150° C. to about 40° C., assuming ambienttemperature of about 25° C. In some embodiments, the membrane and/or thesubstrate may actively be refrigerated (e.g., by blowing cool air orcontacting with a heat sink / cooled surface) in a region spanning fromnip 156 to roller 170, which can be referred to as the “pre-peelingregion”. For example, while the substrate and membrane are in contactwith a heat sink, cooling may be performed to the predeterminedtemperature (e.g., ambient temperature or any temperature below thesoftening point temperature of adhesives and below the softeningtemperature of the membrane). Such cooling may take, for instance, abouthalf a second to about a second and a half, to cool down the membraneand its underlying substrate by about 88% of the difference between theinitial temperature and ambient temperature, e.g., to cool down fromabout 150° C. to about 40° C., assuming ambient temperature of about 25°C.

A reduction of temperature in the pre-peeling region, e.g.,spontaneously or by active refrigeration, as compared to a temperaturethat may have been applied during transfer at nip 156, can, in someembodiments, accelerate or otherwise facilitate the adhesion of the baseof the pattern to the substrate. For instance, cooling may acceleratethe hardening of hot melt adhesives, accelerating the formation ofstronger structures within the grooves and at the interface with thesubstrate. The duration of time that it would take for a leading edge ofthe membrane to traverse the pre-peeling region depends on the speed ofthe membrane and can be termed a “pre-peeling period”. The existence ofa pre-peeling period may, in some embodiments, preclude the need for anyadhesive coating over the surface of the membrane and the substrate.Nevertheless, if an adhesive coating is desired on one or both of thesurfaces of the membrane and the substrate in contact in the pre-peelingregion, in order to facilitate adhesion to the substrate and/orextractability of the pattern, such adhesive could be applied as thinlayers, e.g., not exceeding 2 μm in total when dry. In some embodimentsin which an adhesive coating is applied to the membrane and/or thesubstrate, the adhesive coating may be dried prior to bringing themembrane into contact with the substrate.

Lines 140′ that remain on the substrate following the peeling of theflexible membrane may not yet be electrically conductive but theelectrically conductive particles in the composition may be sintered,fused, or otherwise transformed into a conductive state. Differentsintering methods exist, including thermal sintering, light inducedsintering, microwave sintering, electrical sintering and chemicalsintering, the elected sintering method being dependent upon thecompositions and preferred process conditions. These may in turn bedictated by the intended end product. Generally sintering is performedby the application of heat to form a conductor pattern matching thepattern on the die roller 102 of FIG. 1A.

If the conductive pattern is applied to a semiconductor wafer (e.g., apolished or unpolished, doped or undoped, silicon wafer optionallyhaving an anti-reflective coating), as is common in the production ofcertain solar cells, merely placing electrical conductors over thesurface of the semiconductor would not suffice to achieve the desiredelectrical connection. In such an embodiment, the composition preferablyadditionally contains a glass frit and is fired to cause the conductorpattern to fuse with the substrate. The molten glass etches thedielectric layer of the semiconductor wafer, while allowing metal ions(e.g., silver) to migrate through it to the substrate (e.g., a siliconwafer or thin-film), producing a conductive path between the surfacemetal pattern and the wafer underneath the dielectric or passivationlayer, if one exists. Such firing may be carried out separately, ormerely as an extension of the step utilized to sinter the electricallyconductive particles. The temperatures and times required for the fusingof the conductors to a semiconductor substrate are however differentfrom those needed for mere sintering. For instance, sintering of theelectrically conductive particles may be carried out at a sinteringtemperature within a range of about 100-150° C. to about 800° C.,whereas firing through the composition pattern so as to form electricalcontact with the substrate can be performed at a firing temperaturewithin a range of about 500° C. to about 900° C., the firing temperaturebeing greater than the sintering temperature.

It is noted, that the relatively low amount of adhesive material in thegrooves (e.g., not exceeding 60% per volume (v. %) of the composition,being less than 50 v. %, or less than 40 v. %) and the relatively lowamount, if any, of adhesive material on the surface of the membraneand/or the substrate (e.g., forming a coating having a dried thicknessnot exceeding 2 μm), allows for a relatively short duration during whichthe substrate and its pattern are fired and sintered (e.g., for drying,burn out of the non-metallic components, such as organic adhesives,sintering of the metallic particles and fire-through contact formation),as compared to methods heavily relying on the presence of such adhesivematerials. Having fewer adhesive compounds to eliminate, a patternprepared according to the present teachings could, for instance, havethermal energy applied to the substrate in temperatures ramping up to700° C. or higher for final sintering, for a duration that does notexceed about two minutes (e.g., instead of an expected duration of tensof minutes or even hours for solar cells produced by methods whereadhesive materials are present in relatively high amounts). For example,the firing and sintering profile could include a temperature risingfirst to about 250° C. (e.g., to eliminate volatile solvents, if any),and then to about 400° C. (e.g., to eliminate adhesives), and finally upto about 700° C. (e.g., to finalize sintering of the metallicparticles), such process being typically performed according to thepresent invention during a duration of two minutes or less. The durationduring which the substrate is subjected to a temperature of 700° C. orhigher may be about 5 seconds. Such a shorter duration firing profilemay enable more solar cells to be produced in a given amount of time(high-throughput), compared to solar cells produced by processes whichinclude a longer duration firing profile (low-throughput).

During the filling of the grooves 110 in the membrane 100 withcomposition 120, the composition may optionally be rendered moreflowable by addition of a solvent, which as above-detailed may be anaqueous or an organic solvent. In such a case, the composition mayshrink as it dries and will not totally fill the grooves. FIG. 3A-3Dschematically show optional steps to solve, and optionally utilize, suchphenomenon.

In FIG. 3A, the grooves are filled with a unique composition 210. Suchsubstantially complete loading of the groove by any composition mayresult from consecutive filling of the grooves, the volume of whichreduces with each drying of each applied volume of the same composition.When the dried composition 210 is level with the surface of the membrane100, the structure shown as 140 a is formed. The number of consecutivefilling cycles necessary to entirely fill a groove may depend upon thecomposition, the groove dimensions and the process operating conditions,but typically do not exceed five cycles, three or four cycles beingpreferred.

The effect of the composition 210 shrinking away from the surface of themembrane 100 upon drying, is better seen in FIG. 3B, in which the dipleft by composition 210, applied and dried in a first step, is filled ina second step, which in the depicted example utilizes a secondcomposition 220. This may be repeated as necessary to ensure that thegrooves 110 are eventually filled to substantially the level of thesurface of the membrane 100. When the dried composition 220 issubstantially level with the surface of the membrane 100, the structureshown as 140 b is formed.

When successive steps and/or cycles are performed to fill the grooves,the constituents of the composition may vary between steps/cycles.

In essence, only the last applied composition is required to havesufficient adhesiveness to adhere to the substrate while in earliersteps the adhesive is only required to hold the electrically conductiveparticles together. Therefore, while in FIG. 3A, the composition 210 maycomprise a relatively potent adhesive, being the sole compositionforming the filled groove 140 a, in FIG. 3B, composition 220 would haveto satisfy this purpose. Composition 210 in the latter figure maytherefore comprise either a “poor adhesive” binder or a potent adhesive.The transfer of an intact composition/line to the substrate may be morechallenging for grooves with higher aspect ratios. Therefore in someembodiments, the adhesive used to hold the electrically conductiveparticles together for high aspect ratios may be more potent than thepotency of an adhesive used to hold electrically conductive particlestogether for grooves with lower aspect ratios.

Additionally, or alternatively, the electrically conductive particles,or blends thereof, used for each composition of each filling repeat maydiffer from filling to filling. In some embodiments, the particles inthe first filling may be smaller than in subsequent fillings (or alarger percentage of the particles may be smaller than in subsequentfillings). In such embodiments, in subsequent fillings the particles maybe larger than in the first filling (or a smaller percentage of theparticles may be smaller than in the first filling). For example, theusage in the first filling of a larger proportion of smaller particlesthan larger particles of light reflective material, or the usage of onlysmaller particles of light reflective material in the first filling,may, upon transfer of the light reflective material to the substrate,reduce the diffusivity of light incident upon the light reflectivematerial, whereas the size of particles in subsequent fillings may haveno or less impact on the diffusivity. In such an example, the firstlayer may include light reflective material, whereas subsequent layersmay not necessarily include material that is light reflective, or mayinclude material that is less light reflective/having larger particlesthan the material in the first layer, because the material in subsequentfillings would in any event have no or less impact on the diffusivity.In other embodiments where higher diffusivity is desired, the firstfilling may not necessarily include material that is light reflective,or may include material that is less light reflective/having relativelylarge particles.

In FIG. 3C, a release coating 230 that separates easily from themembrane 100 is used to line the grooves before they are filled with acomposition containing electrically conductive particles, to form thefilled groove cross-section shown at 140 c. Of course, this couldalternatively be a multi-layer structure having additional furthercomposition layers with a higher concentration of adhesive by way ofexample, such as the composition 220. Hence, in some embodiments theconductive lines that can be obtained by the present method may have amulti-layered structure. As noted above, the intact transfer of acomposition/line from grooves having higher aspect ratios may be morechallenging than the intact transfer of the composition from grooveshaving lower aspect ratios. Usage of a release coating may assist in theintact transfer of the composition/line, especially for grooves withhigher aspect ratios.

A release coating 230 may consist of a relatively diluted polymer, in anorganic solvent (e.g., a C1-C7 alcohol such as butanol). The “release”polymer has poor adhesion to the surface of the membrane and preferablyhas a relatively low ash content, allowing for its substantiallycomplete elimination (e.g., by combustion) at a later stage, if it istransferred with the metal lines. In some cases, the polymer of therelease composition can be compatible (e.g., adhesion wise) with thecompositions to be poured thereupon. Thus, the release composition, oncedried, may either remain attached to the grooves' walls or transfer withthe composition lines.

The “release” polymer can be, for example, a water-insolubleethylcellulose polymer or a water-soluble cellulose ether, depending onthe preferred vehicle of such a release coating. The polymerconcentration typically does not exceed 10 wt. %, compositionsconsisting of 5 wt. % or less being favored. As other compositions, therelease composition can be applied to fill the grooves with a squeegee,the organic solvent being eliminated by drying. In view of the lowpolymer content, a dried release composition generally forms upon thewalls of the grooves as a film of 1-2 μm or less. If desired, a secondlayer of release composition can be similarly applied.

The release coating 230 may for example consist 5 wt. % water-insolubleethyl cellulose (such as available as Ethocel™ Std 100, of The DowChemical Company) in 1-butanol (Sigma Aldrich). Alternatively, ifaqueous compositions are preferred, the release coating may consist of10 wt. % water soluble cellulose ether (such as available as Methocel™E15, of The Dow Chemical Company) in deionized water.

It is to be noted that when the compositions used for the loading of thegrooves, as well as for the optional pre-coating with a release coatingand/or post-coating with an adhesive coating are all aqueous (e.g.,water constituting at least 60 wt. % of a liquid carrier of thecomposition), it can be desirable to treat the flexible membrane(generally hydrophobic) to facilitate its even wetting by thesecompositions. Such a treatment, which can be achieved by corona, can bepreferably performed once the release coating, if present, has beenapplied and dried within the grooves, but can also be performed, if atall, before the application of a release coating.

FIG. 3D shows an optional structure 140 d that in addition to thecomposition 210 filling the grooves has an adhesive coating 250 thatcovers the entire surface of the membrane. The adhesive coating 250 isintended to assist in the transfer the composition from the grooves tothe substrate. The surface of the substrate can be, if desired,subsequently washed with a solvent to remove the adhesive from all theareas where it is exposed and does not lie beneath the lines ofcomposition. In most cases, however, removing the adhesive coating bythe use a solvent may prove unnecessary as it will be burned away uponsintering (and/or further firing) of the metal lines transferred to thesubstrate.

Though the structure 140 d is shown in FIG. 3D as comprising a singlecomposition 210, it may be formed from of multiple layers as describedwith reference to FIG. 3B and FIG. 3C.

An adhesive coating 250 may consist of the same potent organic adhesivesas previously mentioned (but be devoid of glass frits), such adhesivesbeing now dispersed or dissolved in a carrier that would not negativelyaffect the structural integrity of the dried compositions filling thegrooves. The amount of adhesive polymer in such an inert carrier can bein the range of 5-50 wt. % of the adhesive coating composition. The“inert” carrier can be at least one of the organic solvents previouslydetailed, the volatility of such solvents shortening the time suchcompositions may affect the previously applied and dried compositions.Subsequent to its drying, the adhesive coating should preferably have athickness within a range of 0.2-3 μm or 0.2-2 μm.

The adhesive coating 250 may for example consist of 5 wt. % polyamidehot melt adhesive (Uni-Rez® 2720), 15 wt. % butanol (Sigma Aldrich) and80 wt. % pentyl acetate (Sigma Aldrich). Alternatively, if aqueouscompositions are preferred, the adhesive coating may consist of 5 wt. %water soluble adhesive made of poly(2-ethyl-2-oxazoline) (such asavailable as Aquazol® 5, by Polymer Chemistry Innovations), 1 wt. %cosolvent, such as butanol, 0.25 wt. % of a first wetting agent, such asa silicone surfactant (as BYK®-349 by BYK), 0.075 wt. % of a secondwetting agent, such as a silicone surfactant (as BYK®-333 by BYK) indeionized water.

Other Electrodes

The processes as described above are better suited for forming thinconductors but when coating a large surface with an electrode isdesired, such as the back electrode of a solar cell by way of example,the process can be considerably simplified as shown in FIGS. 4A to 4C.In FIG. 4A, a doctor blade having a rounded tip is pressed against themembrane 100 as it passes between a pressure roller 402 and the doctorblade 400. An excess of composition 404 is applied to the membrane 100upstream the doctor blade 400 and its thickness is reduced uniformly asit passes beneath the doctor blade. The thickness can be set by varyingthe radius of curvature of the tip of the doctor blade 400 and theapplied pressure. The excess composition need not be applied along theentire length of the doctor blade or the doctor blade may be optionally“segmented” the composition being layered within a desired portion ofthe membrane. The composition 404 can then be dried.

The membrane 100 carrying dried composition 404 is then pressed againstthe substrate 150 as shown in FIG. 4B by passing it through the nipbetween two pressure rollers 406 and 408 (e.g., having an outer surfacehardness within a range of 10 shore 00 to 70 shore A). Pressure rollers406 and 408 can be further heated to facilitate the transfer ofcompositions including an adhesive having a softening point at transfertemperature. The membrane 100 can then be peeled away from the substrate150 as shown in FIG. 4C to leave a coating of the composition 404 on thesubstrate.

Between the time that membrane 100 and substrate 150 pass through thenip, and the time that membrane 100 is peeled away from substrate 150,membrane 100 and/or underlying substrate 150 may cool down spontaneouslyor by active refrigeration. The coating of dried composition 404 canthen be sintered (e.g., heated to a temperature of 700° C. or higher) torender it conductive onto the substrate 150 and if necessary, fired toform the back electrode of the solar cell. In some embodiments, theduration of the application of thermal energy may not exceed twominutes. A suitable solar cell back electrode may not only be fused tothe wafer as previously explained, but can also create a highly dopedlayer (e.g., aluminum doped) on the silicon substrate. Such highly dopedlayer, called a back scattering field (BSF), can improve the energyconversion efficiency of the solar cell. Such considerations are knownto the skilled persons, who can accordingly formulate composition 404following principles previously detailed for composition 120.

It will be appreciated that the laminating steps shown in FIG. 1C andFIG. 4B can be carried out at the same time. This enables a large areaback electrode to be applied to a semiconductor wafer at the same timeas a conductive grid pattern is applied to its front surface. Thesintering and fusing of the composition on both sides of the wafer maythen be conducted at the same time.

Back electrodes can alternatively be applied to the opposite side of thesubstrate by any method conventionally used in the industry ofrelevance, for instance by screen printing. Such method may be useddirectly on the back side of the substrate, but preferably may serve toapply a layer of compositions of the present disclosure on a flexiblemembrane allowing the concomitant transfer of “front” metal pattern and“back” electrode according to the above-detailed method. If forming theback electrode on a membrane, gravure printing can be additionallyconsidered to form the layer of composition on the membrane.

Example of Solar Cell

A conductive metal pattern was prepared according to the aboveprinciples, schematically summarized in FIG. 5A. In the figure, a metalpattern 500 is illustrated. Such a pattern can be “negative”, below thesurface of the membrane when the dried compositions substantially fillthe grooves, or “positive” protruding above the substrate (e.g., wafersurface) after transfer. The longitudinal grooves of metal lines 510result from rules 106 (see, FIG. 1A or FIG. 2A-2B), whereas transversegrooves or metal lines (e.g., bus bars) are shown as 520, if“uninterrupted” traversing all longitudinal lines of the metal pattern,or 530, if traversing only a subset of lines 510. In any event,transverse grooves or lines 520 and 530 result from rules 108 (see, FIG.1A or FIG. 2A). The longitudinal lines due to collect the currentgenerated by the intended photovoltaic cell may also be referred to asgrid lines or fingers. For simplicity, any electrical conductor otherthan the grid lines in an electrically conductive pattern prepared bythe present method can be considered as a “connector” regardless ofshape and elements being connectable therewith. As mentioned, such aconnector may serve as an “intra-connector” connecting at least part ofthe grid lines of the electrically conductive pattern one with another(e.g., bus bars), or may serve as an “inter-connector” connecting atleast part of the electrically conductive pattern to an externalcircuitry or a separate electrically conductive pattern. The two typesof connectors are not mutually exclusive and an intra-connector mayadditionally serve as an inter-connector. For instance, a bus bar mayserve to connect grid lines of an electrically conductive pattern on onesolar substrate and to connect the same with a distinct conductivepattern on a different solar substrate. Therefore, a pattern ofelectrical conductors, and an electrically conductive pattern resultingfrom the sintering and optional firing of the same following applicationto a substrate, may include grid lines and may optionally includeconnectors. In some embodiment, connectors may be applied by anysuitable method to a substrate following transfer by the present methodof electrical conductors due to serve as grid lines and before sinteringof the same.

The flexible membrane used in the present example was a castpolypropylene film (50 μm thick CPP; RollCast™ 14 of R.O.P. Ltd). It waspatterned with a rotatable die 102 wherein a nickel shim provided for aseries of about 85 rules 106 for the formation of the longitudinalgrooves. The edges of rules 106 were trapezoidal, as shown in FIG. 5B,with a base having a width W_(B) of 20 μm, a flat top having a widthw_(T) of 12 μm, and a height h of 32 μm. The distance d between adjacentrules was of 1.8 mm. The length of each rules along the circumference ofthe roller 102 was set to be compatible with the solid support to whichthe metal pattern would be transferred (including “margins” in betweenpatterns, if desired). The rotatable die 102 formed a patterning nipwith a counter die (e.g., cylinder 104), the pattern of grooves beingformed as the membrane passed through the nip.

The grooves so formed (having a base W_(B) of about 25 μm, a top w_(T)of 12 μm and a depth (height h) of about 25 μm, the distance between thefacing edges of two adjacent grooves being of about 1,775 μm) werefilled with a release composition consisting of 5 wt. % Ethocel™ Std.100 (The Dow Chemical Company) in 1-butanol (Sigma Aldrich). Thecomposition was applied to the patterned membrane using a doctor blade(MDC Longlife Multiblade, Daetwyler) positioned at an angle of 65-70° tothe surface normal. The stainless steel beveled blade had a width of 20mm, a thickness of 0.2 mm, a bevel angle of 3°, a bevel length of 2.7 mmand a tip radius of 18 μm. The force used was of 3N/cm. The doctor bladewas parallel to the axis of rotating die 102, perpendicular to thegrooves generated by rule 106, and in relative motion with the flexiblemembrane, the moving direction being parallel to lines 510. The membranewas then heated with a hot air gun until the release composition becamea substantially dry film. The heating/drying temperature was selected toprevent membrane deformation (<70° C. for CPP). In the present example,the release composition was applied in a single step.

After the formation of a thin release film coating the walls of thegrooves, a metal paste was loaded within the grooves. The metal pasteincluded a) silver particles (1-3 μm beads) and lead oxide based glassfrits (1-3 μm chunks), the foregoing constituting about 60 vol % of thesolids, glass frits being present at approximately 3-5 vol % of theelectrically conductive particles; b) a polyamide adhesive, Uni-Rez®2720 or Uni-Rez® 147, constituting about 40 vol % of the solids; andpentanol, constituting 40 vol % of the total paste. When converted towt. % of the total composition, such metal paste was composed of87.3-88.7 wt. % silver particles, 1.3-2.7 wt. % glass frits, 6 wt. %adhesive and 4 wt. % carrier.

Each loading step of the metal paste was followed by a substantialelimination of the organic carrier and drying of the composition with ahot air gun. It typically took three passes to fully fill the grooves,level with the surface of the flexible membrane.

If an additional adhesive coating layer was desired, it was applied by a6 μm wire bar, the dried adhesive coating having a thickness of lessthan 2 μm. The adhesive coating applied in some experiments included 10wt. % of Uni-Rez® 2720, in a mixture of solvents consisting of butanoland pentyl acetate (at a weight per weight ratio of 1:5). Alternativeadhesive coatings were prepared using additional polyamide hot meltadhesives including Uni-Rez® 2620 of Kraton Corporation, USA, Macromelt®6211, Macromelt® 6224, Macromelt® 6238 and Macromelt® 6239, of Henkel,Germany, and Versamid® 744 and Versamid® 754 of Gabriel PerformanceProducts, USA. 10 wt. % of each polyamide hot melt adhesive was mixedwith 90 wt. % of organic solvents. The Macromelt® and Versamid®polyamide adhesives were blended in propylene glycol methyl ether (suchas commercially available as Dowanol™ PM from the Dow Chemical Company).The Uni-Rez® 2620 and Uni-Rez® 2720 polyamide adhesives were applied infurther additional solvents, each having been blended in (a) 90 wt. %butanol, (b) 90 wt. % pentanol, (c) 15 wt. % butanol and 75 wt. % amylacetate and (d) 15 wt. % pentanol and 75 wt. % amyl acetate; allsolvents being supplied by Sigma-Aldrich at purity level of at least95%. All afore-said adhesive coating compositions based on polyamide hotmelt adhesives were found satisfactory for the transfer of patterns fromthe membrane to the substrate.

In an additional series of experiments, different adhesive polymers weretested and found similarly suitable. The three further alternativeadhesive coatings consisted of (a) 20 wt. % of a fully hydrogenated gumRosin (Foral™ AX-E by Eastman Chemical Company) in 80 wt. % Dowanol™ PM,and (b) 10 wt. % of a terpene phenolic resin (Sylvaprint® 3523 orSylvaprint® 7002 by Arizona Chemical) in 90 wt. % Dowanol™ PM.

FIG. 5C shows a micrograph picture taken by confocal laser scanningmicroscope showing a groove of a flexible substrate filled with metalpaste (see for example 140 in FIG. 1A). As can be seen, the groove issubstantially filled to the level of the surface of the flexiblemembrane, while the areas surrounding it are substantially free of driedcomposition. For such picture, an adhesive coating was omitted.

The flexible membrane including the patterned grooves filled with drycomposition was contacted with a textured boron-doped silicon waferhaving a phosphor doped emitter side and an anti-reflective coating ofsilicon nitride. Such wafers may assume a variety of sizes (e.g.,156×156 mm or 125×125 mm) and thicknesses (e.g., in the range of 150-300μm), the ones used in the present example having a square shape of about156 mm side and a thickness of about 200 μm. The membrane was pressedagainst the wafer at a pressure of 6Kg/cm², by passing through a nip 156at a speed of 5 cm/s. Pressure rollers 152 and 154 were heated to about130-140° C. The adhesive coating, if any, flowed into the Si wafersurface texture, to which it adhered after cooling of the wafer back toabout ambient temperature. The CPP membrane was then peeled off thewafer, while the coating layer of adhesive and the metal patterntransferred to the wafer and remained thereon following the removal ofthe membrane. The transferred metal patterns were sintered and fired ata temperature profile reaching a peak temperature of about 750° C. in asolar belt furnace Despatch CDF-SL. The patterns so treated were at atemperature of 700° C. or higher typically for at least 5 to 20 seconds,before being cooled back to ambient temperature.

The normalized contact resistance (Rc) of the resulting conductive lines(e.g., of the grid lines) was measured with the Transfer Length Method(TLM). Rc values were between 0.05 to 0.1 Ω.cm (Ohm.centimeter), valuesbelow 0.2 Ω.cm being considered highly satisfactory, representing a lossof efficiency of less than 0.1% as compared to an optimal contact havinga null Rc and values between 0.2 Ω.cm and 0.3 Ω.cm being deemedacceptable. It is thought that such an advantageously good normalizedcontact resistance results, inter alia, from the relatively low amountof adhesive materials used in the process of the present disclosure. Fora sheet resistance of the substrate (e.g., a silicon wafer) between 50and 110 Ω/□ (Ohm per square or Ω/sq), and conductive lines having awidth of at least 15 μm and a height of at least 8 μm, it is expectedthat the normalized contact resistance will be 0.3 Ω.cm or less, 0.25Ω.cm or less, 0.2 Ω.cm or less, 0.15 Ω.cm or less, 0.1 Ω.cm or less, or0.08 Ω.cm or less. For a sheet resistance of the substrate between 111and 140 Ω/□ and conductive lines having a width of at least 15 μm and aheight of at least 8 μm, it is expected that the normalized contactresistance will be 0.6 Ω.cm or less, 0.5 Ω.cm or less, 0.4 Ω.cm or less,0.3 Ω.cm or less, 0.2 Ω.cm or less, or 0.16 Ω.cm or less. A review ofcontact resistance and methods of measuring the same with respect tosolar cells is available, inter alia, in “Solar cell contactresistance—a review” by Dieter K. Schroder published in IEEE Transactionon Electron Devices, Volume ED-31, Issue No. 5, May 1984.

FIG. 5D shows a micrograph picture taken by confocal laser scanningmicroscope (LEXT OLS4000 3D of Olympus Corporation) showing a contactline made of metal paste (as filled in the groove shown in FIG. 5C)transferred to a wafer (see for example 140′ in FIG. 1D) and sintered asabove-described. As seen from the figure, the sintered metal linesubstantially retained the shape of the rules defining the grooveswherein it was prepared.

It is an advantage of the process of the present disclosure that theconductors can be very narrow (e.g., ˜20-25 μm or less, half or lessthan typical art values), while retaining a sufficient height, as can beassessed by a relatively high aspect ratio, ASP being about 1:1 in thepresent example.

A solar cell was similarly prepared, using the same membrane and patternof grooves, by replacing the above described non-aqueous compositions byaqueous ones. The release coating was prepared using a releasecomposition including 10 wt. % Methocel™ E15 (The Dow Chemical Company)in deionized water. The release composition was twice applied and dried,following which the grooved side of membrane was exposed to close coronatreatment (BD-20AC Laboratory Corona Treater, by Electro-TechnicProducts).

A water-based metal paste was then loaded within the grooves. The metalpaste included a) silver particles (1-3 μm beads) and lead oxide basedglass frits (1-3 Ω.am chunks), the foregoing constituting about 60 vol %of the solids, glass frits being present at approximately 3-5 vol % ofthe electrically conductive particles; b) a poly(2-ethyl-2-oxazoline)adhesive, Aquazol® 5, constituting about 40 vol % of the solids; anddeionized water, constituting 40 vol % of the total paste. Whenconverted to wt. % of the total composition, such metal paste wascomposed of 87.3-88.7 wt. % silver particles, 1.3-2.7 wt. % glass frits,6 wt. % adhesive and 4 wt. % carrier. The aqueous metal pasted wasloaded in six filling/drying steps.

Finally, a water based adhesive coating was applied to the entiresurface of the loaded membrane. The aqueous adhesive coating consistedof 5 wt. % Aquazol® 5 (a poly(2-ethyl-2-oxazoline) by Polymer ChemistryInnovations), 1 wt. % butanol, 0.25 wt. % BYK®-349 and 0.075 wt. %BYK®-333 (both wetting agents by BYK), all in deionized water.

Light Reflective Grid Lines

When considering the particular use of the present method for thepreparation of a flexible membrane used for the manufacturing of solarcells, optional additional steps may be further implemented. As known topersons skilled in the fabrication of photovoltaic devices, a recurrentproblem is the trade-off between the number and size of the grid linesand the reduction in the amount of photo-current that can be generateddue to their shading.

The shading of the light collection surface of such devices by the gridsof conductive lines (fingers and bus lines) may amount to up to about 5to 10% of the surface which can impair the resulting efficiency ofphotovoltaic conversion. Light incident upon the conductive lines mayreflect back to the environment, this energy being lost for theunobstructed light collecting areas adjacent to the grid lines. Properselection of the rules forming the groove profile within the flexiblemembrane (hence affecting the resulting contour and size of theconductive lines that can be prepared therein) may reduce such shading.For instance, the shape of the rules (and conductive lines which mayresult therefrom) may enhance or facilitate the reflection orredirection of impinging light onto the non-obstructed photovoltaiclight harvesting surface of the future substrate (the “active area”),thus decreasing the effective optical shading of the grid. For similarreasons, it is expected that fingers and bus lines having a relativelysmooth surface provide for a higher reflectivity/lower light diffusivitythan otherwise similar conductive lines having a relatively roughersurface. A lower diffusivity of the conductive lines when their slopeswould otherwise suitably redirect the incident light to the active area(e.g., forming an angle of 45° or more) of the light harvestingsubstrate may also decrease the shading effect of the grid, a higherportion of incident light being properly redirected. Conversely,conductive lines having shapes and/or slopes which would otherwisereflect incident light away from the available substrate area maybenefit from a higher diffusivity which may increase the amount of lightbeing redirected to the photovoltaic surface free of grid lines.Preferably, the shape, size and/or spacing of therules/grooves/conductive lines facilitate or enhance redirecting thelight to active areas in response to a wide range of incident lightangles. However, the ability to utilise light incident from a wide rangeof angles may not be essential if solar panels including such cells areequipped with a tracking mechanism following the movement of the sun,limiting the ranges of incident light angle that would impinge on theconductive lines and maintaining sufficient active area of photovoltaicsurface as day proceeds.

The volume bounded by any pair of reflective grid lines may beconsidered to act as a light funnel which may reduce the effect ofactual shading, by increasing the exposure of the photovoltaic area freeof grid lines to light, enhancing the efficiency of the solar cell or ofa solar panel formed by an interconnected plurality of such cells. Theshape of this “funnel” is derived from the shapes of the conductivelines bordering it and the profile of their reflective surfaces. Asabove explained, the faces of a conductive line may have a range oflight reflective or diffusive properties, while remaining suitable fortheir intended use. Thus, the terms “reflective surface(s)” or“reflective face(s)” of a grid line do not intend to limit such surfacesto ideal mirror-like walls.

It can be theoretically demonstrated, using principles of ray optics,that for many illumination conditions, the higher the angle between thereflective surface of the grid lines and the solar cell surface (0°being parallel to the wafer surface and 90° being perpendicular) thehigher the amount of light that may be reflected back to the solar cellnon-obstructed area. Taking for example an ideal mirror-like inversed Vshaped grid line having substantially smooth and perfectly specularreflective faces essentially lacking any diffusive reflection forming anangle of less than 45° with the solar cell light harvesting surface, anincident light beam perpendicular to the solar cell surface will bereflected back (e.g., to the air), and will not contribute tophotovoltaic conversion and current generation, resulting in effectiveoptical shading that is equivalent to the geometrical shading. Incontrast, a similarly shaped line having ideally reflective mirror-likefaces forming an angle of more than 45° will totally reflect an incidentperpendicular light beam to the solar cell surface, resulting in nulleffective optical shading.

The above depicts a very particular situation and when calculating lightlosses for standard solar cell operation conditions, the followingfactors should be considered:

-   -   1. The losses should be integrated for various light incident        angles, and intensities, according to the illumination        conditions as day or seasons proceed.    -   2. The solar cells are usually encapsulated (e.g., in glass and        adhesive) for protection, which encapsulating material(s) may        change the optical light path and intensity of the incident and        reflected beams. Light beams might be reflected from the grid        surface to the protective capsule—air interface, go through        total internal reflection and be redirected to the solar cell        surface.    -   3. The surface of the lines of the solar grid is seldom ideally        smooth and light is usually reflected in a diffusive way. The        spatial distribution and intensity of the diffracted light at        the various incident angles may depend on the roughness of the        surface.

The theoretical light loses considering the above factors can becalculated by ray optics, suggesting that the effective optical shadingcan be reduced as the angle of the faces of the conductive lines withrespect to the solar harvesting surface is increased. For faces formingan angle greater than 45°, the effective optical shading is of less than30% of the actual geometrical shading for totally diffusive surface(Lambertian surface), such shading decreasing as the angle increases.When diffusivity of the lines surfaces is decreased (e.g., by reducingroughness/increasing smoothness), the effective optical shading can befurther decreased, down to 0% for ideal mirror-like surfaces as previousexplained. Any of the above approaches to reduce shading of theconductive lines, including by selecting a favorable profile to thelines and/or, depending on the angle formed by the reflective surface,by increasing reflectivity of incident light towards the active area ofthe harvesting substrate and/or by decreasing diffusivity of the lightaway from the wafer active area is desirable to increase thephotovoltaic conversion efficiency of a solar cell prepared using aflexible membrane according to the present teachings.

Based on the above principles, a rule/groove/conductive line cansuitably have a trapezoidal profile, preferably with a small flat topwidth w_(T) to lessen light loss (e.g., having a w_(T) of 5 μm or less)or have a triangular profile. The base for such profiles canadvantageously have a width W_(B) between about 10 μm and about 25 μm.When the grid is to be used in association with a suitable protectivecase which may redirect at least some of the light back to the activelight harvesting area of a solar substrate, the angle between the faceof the line and the substrate (represented by a in FIG. 5B) can be atleast 25°, at least 30°, at least 40°, at least 45°, at least 50°, atleast 60°, or at least at least 70°. In absence of furtherencapsulation, the faces of the conductive lines are preferablyrelatively poorly or essentially not diffusive, the total lack ofdiffusivity afforded by mirror-like surfaces being particularlyadvantageous. In such case, the angle between the faces of such highlyspecular reflective/poorly diffusive lines and the substrate can be atleast 60°, at least 65°, or at least 70°. Whether or not encapsulated,the angle between the face of such a conductive line and the substrateis of at most 90°, at most 85°, at most 82.5°, at most 80°, at most77.5°, or at most 75°. Advantageously, the angle formed by the face ofthe conductive line and the substrate is between 60° and 85°, or in therange of 65° to 82.5° or 70° to 80°. When the conductive lines are toserve in applications other than the preparation of solar cells, inparticular with light reflective faces, additional angles between thesubstrate and the face of the line raising therefrom can be suitable.Hence, in some embodiments, this angle can be within the range from 20°to 90°.

It will be appreciated, that while the angle formed by a side of thegroove with respect to the surface of the membrane can be designed to beconstant until the apex of a cross-section profile of the groove in theflexible membrane before loading, such angles may mildly vary during theprocess, in particular once the conductive pattern is transferred to itssubstrate and sintered thereon to form an end-product solar cell. Forexample, assuming a groove having a triangular cross-section profile,the faces of the triangle forming ab initio an angle of 45° with thesurface of the membrane, a conductive line of a solar cell resultingfrom such a groove might have a less perfect triangular profile.Considering only half of its profile, from the base of the solar cellsubstrate to the top of the conductive line (e.g., where the profile issymmetric on opposite sides of an axis traversing from the base to thetop), the angle in a first segment can be either less than 45°, theprofile of the line having an initial shallow slope, or on the contraryforming an angle of more than 45°, the profile of the line having aninitial “stair” slope, such angles further varying as the half profileof the line reaches its apex. In some embodiments, the layer or layersof the conductive line may follow the profile of the cross section ofthe conductive line, regardless of whether the conductive line includesone layer, two or more layers of the same sintered material, or two ormore layers of different sintered materials. The profile can be mildlyconvex or concave, or with alternating convex, concave and straightsegments. To take into account the profiles of the resulting conductivelines, which may form a variable angle as considered at different pointsalong the slope formed between the basis (or in other words, base) andthe top of the line, it can be preferred to consider the average of theangle as the mean of tan⁻¹ of the profile slope (=derivative) at eachsub-segment along the tapering face. Such value can also be referred toas the averaged slope. Using such method, and assuming that in theafore-detailed illustrative example, the angles of the triangularoidshape do average to 45°, even if fluctuating below and above this valuealong different sub-segments, then the averaged slope of thehalf-profile would be of 1.

Hence in some embodiments, the averaged slope of one side of aconductive line elevating from the base of the solar cell substrate tothe apex of the line (i.e. a tapering face) can be at least 0.50 (˜26°),at least 0.75 (˜37°), at least 0.85 (˜40°), or at least 1 (45°), or atleast 1.15 (˜49°), at least 1.3 (˜53°), at least 1.4 (˜54°), at least1.5 (˜56°), or at least 1.7 (˜60°). In some embodiments, the averagedslope is at most 10 (˜84°), at most 8 (˜83°), at most 6 (˜80°), at most4 (˜76°) or at most 2 (˜63°). For example, an averaged slope range of1.4 to 10, corresponds to an aspect ratio range of 0.7:1 to 5:1.

As mentioned above, light beam perpendicular to the solar cell surfacethat is incident on a conductive line having perfectly specularreflective faces essentially lacking any diffusive reflection forming anangle of less than 45° with the solar cell light harvesting surface,will be reflected back (e.g., to the air), and will not contribute tophotovoltaic conversion and current generation, resulting in effectiveoptical shading that is equivalent to the geometrical shading.Similarly, it is expected that a segment of a profile of a conductiveline having slope less than 1 (<45°) would reflect a light beam,perpendicular to the solar cell surface, that strikes the segment, awayfrom the solar cell non-obstructed area. For a conductive line (e.g.,grid line) produced in accordance to the present invention, the sum ofthe lengths of such segments along the base of the profile having slopeless than 1 (the segment being inclined to the plane of the substrate atan angle of less than)45° is at most 15 μm (e.g., 12 μm or less, or 10μm or less).

Such sum of lengths can be calculated, for instance, by dividing aprofile having a base width of X μm along an x-direction in X segmentseach having a length of 1 μm. The slope of each segment can be definedby the absolute value of the difference between the heights of theprofiles at each end of the segment (e.g., Δy=|y₂−y₁|, wherein thepositions of y₁ and y₂ are measured in micrometres along a y-directionorthogonal to the base of the profile). For each segment, the slope canbe classified as either being of less or more than 1 (the segment beinginclined to the plane of the substrate at an angle of less or more than45°). The number of segments having a slope of less than 1 can be summedup for all X segments. Taking for illustration a hypothetic profilehaving a width of 30 μm and a rectangular shape, in fact all 30 segmentsof this profile are flat (form an angle of 0° with respect to the baseof the profile) so that the sum of the dimensions of such segments ofthe profile having slope less than 1 is 30 μm. Assuming, in contrast, aperfect triangle, each of its faces forming an angle of more than 45°with the base of the profile, then the sum of the dimensions of suchsegments of the profile having slope less than 1 is 0 μm.

In addition, or alternatively, to the selection of a more favorableshape for the cross-section of a conductive line, a light modifyingcoating can be applied to the conductive lines in a manner similar tothe manner described for the optional release layer. As used herein, theterm “light modifying” relates to any type of coating that may reduceshading or increase photovoltaic harvesting and conversion. The coatingmay perform any of the following: increase smoothness or decreaseroughness of the conductive line, increase reflectivity of incidentlight towards the active area (whether directly or indirectly as aresult of an optional protective case) or decrease the diffusivity ofthe light away from the active area.

In some embodiments, the light modifying coating may be applied to asurface of the membrane prior to the loading of a composition into thegrooves. If a release coating is to be applied, then the light modifyingcoating may be applied after the application of the release coating.After the light modifying coating has been applied, the surface of themembrane may be wiped so that the light modifying coating remainssubstantially only within the grooves. The light modifying coating maybe dried, after the wiping, so that the dried light modifying coatingremains only on the walls of the grooves.

For example, a light modifying coating such as a reflectivity-enhancingor diffusivity-reducing composition including a light reflectivematerial (e.g., including relatively small particles of a lightreflective material), may be used to coat the walls of the grooves, sothat upon transfer to the substrate, the transferred lines are coatedwith a thin layer of the reflective material, so as to reduce or preventshading.

Example of Pattern with Light Reflective Faces

The flexible membrane used in the present example was theabove-mentioned cast polypropylene film (50 μm thick CPP; RollCast™ 14)patterned with a rotatable die 102 forming a patterning nip with acounter die (e.g., cylinder 104), the profile of rules 106 beingtrapezoidal, with a base having a width W_(B) of 20 μm, a flat tophaving a width w_(T) of 12 μm, and a height h of 32 μm. Such profileprovides for grooves having inclined walls ideally forming a steep angleof about 83 ° with the surface of the film, however as the actualdimensions of the cross section of a resulting groove and subsequentlongitudinal line mildly diverge yielding measurements closer to a basewidth W_(B) of 25 μm, a top width w_(T) of 12 μm and a height h of 25μm, the angle between the reflective surfaces and their substrate is infact of about 75°. Such angle, calculated based on the sizes achievedwithin the flexible membrane, may slightly decrease following subsequentsteps of lamination, sintering and firing.

The grooves so formed, being separated from one another by a distance dof about 1.8 mm, were filled with a release composition consisting of 5wt. % Ethocel™ Std. 100 in 1-butanol, applied to the patterned membraneand subsequently dried as described above.

After the formation of a thin release film coating the walls of thegrooves, a layer of nano silver ink containing about 40 wt. % silverbeads having a particle size of less than 100 nm was deposited withinthe grooves. Non-limiting examples of suitable nano silver inks includeNano silver ink Metalon® JS-A101 and JS-A102 from NovaCentrix® and thereare numerous commercially available sources of metal nano inks havingaverage particle size (e.g., z-avg particle size as determined by DLS)between about 40 nm and 200 nm or between about 30 nm and 100 nm thatcan be suitable. The nano silver ink was deposited on top of the releasefilm already coating the grooves with a doctor blade by the same methodas described above and dried for approximately 10 minutes with a hot airgun to form a thin nano silver layer.

To the grooves now thinly coated with a release film contacting theflexible membrane and a nano silver layer, the core of the conductiveline was added. The interior of the line was prepared as above-detailedby 3 steps of metal paste deposition followed by the application of anadhesive coating layer. The filled grooves were transferred to the Siwafer by lamination, sintered and fired in the same way described forprevious example.

A control was similarly prepared, lacking the nano silver layer. Thefired grid of the lines having the nano silver layer displayed a lowerlight diffusivity as compared to the grid prepared without the nanosilver ink layer. Such findings were established by independent trainedobservers by tilting at least five grids of each type against a constantlight source, the relative diffusivity of the grids being ranked in ablinded manner. It was therefore assumed that the nano silver layerenhanced the smoothness/decreased the roughness of the exterior faces ofthe conductive lines, an hypothesis confirmed by microscopic study ofthe reflective faces of the lines using a confocal laser scanningmicroscope (Olympus® LEXT OLS4000 3D).

While in the present example, the nano silver ink was applied to thegrooves after the formation of a release layer, this earlier step is notdeemed essential and a suitably formulated ink may satisfactorilyachieve release of the conductive lines from the flexible membrane uponfuture lamination. It is believed that in addition to theabove-exemplified silver, metal nano inks including aluminum, chrome,cobalt, copper, gold, indium tin oxide, molybdenum, nickel, palladiumand platinum may similarly increase grids' reflectivity (or decreasetheir diffusivity). While the material improving the reflectance of thefaces of the lines can be a conductive metal, alloy or metal oxide, thisis not essential, the reflective enhancing layer, if applied,predominantly serving to reduce the shading and improve the efficiencyof a resulting solar cell, the ability of the line to conduct electricalcurrent being predominantly provided by its core.

The light reflective material can be made of silver nano particles. Itcan be present in the reflective composition in amount of 1 to 20 vol %.Once dried, the light reflective coating may have a thickness in therange of 0.2 to 10 μm.

Plurality of Sets of Grooves

In the embodiments of the invention previously detailed, there wasdescribed a flexible membrane in which one pattern of grooves wasformed, then loaded with the compositions of interest ahead oflamination upon the substrate of choice for transfer of the pattern ofdried composition thereto. If distinct patterns (e.g., having differentshapes, different dimensions, different profiles, different compositionsand the like differences) are to be formed on a same substrate, themethod according to the present teachings may be suitable. For example,a first membrane with a first pattern can be applied on a first surfaceof the substrate, while a second membrane with a second pattern can beapplied on a second surface of the substrate. The first and secondsurface of the substrate can be on the same side of the substrate,typically in non-overlapping areas, but can also be on opposite sides.Moreover, each pattern in each membrane may be loaded with same ordifferent compositions.

By way of example, in a solar cell the conductive lines of the lightharvesting side (the solar face) can be formed by transfer of a firstpattern of a first composition from a first membrane and the backelectrode can be formed by transfer of a second pattern of a secondcomposition from a second membrane. Preferably, the distinct patternsare concomitantly transferred during the lamination step in whichpressure is simultaneously applied on both membranes each facing itsrespective face of the substrate.

Distinct patterns which are to be transferred to a same side of asubstrate can advantageously be obtained using a single membrane asexplained in the following. FIG. 6 represents a membrane having aplurality of sets of grooves, each filled with a different material.Such membrane may be utilized for providing differing patterns ofmaterials on the substrate. By way of example, the first pattern mayprovide for the formation of the bus bars, while the second pattern maypermit the preparation of the fingers (i.e. grid lines). The twopatterns of grooves may each partially correspond to the desired patternto be applied to the substrate. In such case, while the two patterns mayeven intersect in some areas, the compositions of the first and secondpattern can differ, for instance, in the amount of glass frits. In thisillustrative example, while the compositions of the second pattern dueto form the grid lines would include glass frits to properlyelectrically contact the substrate in the finished solar cell, thecompositions of the first pattern due to form the bus lines would notneed as high amounts of glass frits. In some embodiments, thecompositions serving for the preparation of the bus lines may even bedevoid of glass frits, as such lines are mainly used to interconnect thegrid lines rather than electrically connect the substrate.

In FIG. 6 a groove pattern symbolized by groove 110A is embossed intomembrane 100A. The first groove pattern is then filled by a firstcomposition 120A, with the aid of a doctor blade 130A, the action ofwhich being backed by pressure roller 132A, or by any other desiredmethod. As described in more detail above, such as in relation to FIGS.1A-1D, and 3A-3D, a plurality of filling steps may be utilized, and aplurality of filling and/or drying stations and/or doctor blades may beutilized (not shown). Similarly, a cleaning station may be optionallyincluded, so as to facilitate the removal of composition in the spacesbetween the grooves.

Once the grooves of the first pattern are filled as shown by way ofexample by 140A, 140B, 140C, etc. a second set of grooves symbolized bygroove 112 is formed in the membrane. The second set of grooves is thenfilled by a second composition 120B utilizing doctor blade 130B, theaction of which being backed by pressure roller 132B, or any otherdesired method. As the grooves of the first pattern 140A, 140B, 140C arealready filled, the second composition 120B is applied to the grooves ofthe second pattern. The second sets of grooves may be filled by a singleor a plurality of filling steps, doctor blades, flowing aids and like,until a desired level of fill 142 is achieved. Thus, the membrane maycarry a plurality of groove patterns and each of the groove patterns mayutilize a composition providing a desired set of characteristics. Whilein this figure, the two patterns of grooves are respectively illustratedby grooves 110A and 112, which may seem parallel one to the other in therepresented view, this need not be the case and the grooves of onepattern may assume any desired position and orientation with respect tothe grooves of the second pattern.

FIG. 6 also shows that different methods may be utilized to form thegrooves. While the first set of grooves is depicted as being formed byembossing by die roller 102A (104A representing a corresponding pressureroller or counter die), the second set of grooves are formed utilizingan alternative method symbolized by the numeral 150. Such groove formingdevice may be one of many devices, ranging from a dedicated punch to alaser source for selectively ablating parts of the membrane 100A or incertain cases for causing membrane shrinkage by heating. Alternatively,the pattern of grooves can be formed on a flexible membrane by apatterning technique, such as photolithography.

Once the membrane grooves are filled with various compositions, anadhesive coating may be applied, as illustrated by 250 seen for examplein FIG. 3D, and the membrane may be brought into contact with thesubstrate for transferring the patterned compositions thereto, asdescribed above.

It will be readily appreciated that the present method may facilitateproper registration between various patterns and the substrate. Thepatterns can be separated from one another on the flexible membrane in amanner that may further facilitate such registration.

FIG. 1E depicts several dashed line elements which reflect process stepsdirected to producing a membrane with a plurality of compositions, forlater transfer to a substrate. After the first pattern grooves arefilled 315, an additional set of grooves is produced 340 on the membraneby any desired manner. The second set of grooves is then loaded 345 witha second composition (the loading being optionally performed in repeatedcycles of filling and drying a set of compositions which differ from thecomposition or set of compositions used to fill the grooves of previouspattern). Those process steps for patterns other than the first patternmay be repeated any desired number of times, as shown by dot-dashed line355. Once all desired patterns have been formed in the membrane andloaded with the desired pastes of materials, the process continues 350to contacting the membrane and subsequent transfer of the materials tothe substrate (which involves steps such as previously described 320,325, and 330), the final step 335 being adapted to sinter the materialsof all transferred patterns. In some embodiments, the steps of theprocess are not performed in a continuous manner nor necessarily by thesame entity. In certain embodiments, material may not be completelysintered, and in some embodiments temperatures and other environmentalconditions are adjusted to prevent damage to the numerous compositions.

Uniformity of Conductive Lines

One of the striking advantages of patterns transferred using flexiblemembranes and methods of the invention relates to the uniformity of thelines. While such property is apparent at numerous stages in theprocess, from the grooves formed in the membrane to the conductive linesfollowing sintering of the transferred paste pattern, through thetransferred lines not yet sintered, parameters supporting suchhomogeneity can be assessed more easily in the finished product, thesintering of the metal paste fixing the dimensions and shape of thelines transferred to the substrate and rendering them conductive.

The uniformity of the conductive lines pertains to a number of factors.First, the attachment of the lines to the underlying substrate occurssubstantially over the entire width and length of the line. Withoutwishing to be bound by any particular theory, it is believed that theconformability of the membrane to the substrate under the conditionsprevalent upon transfer (e.g., temperature, pressure, etc.) enablescontact intimate enough to allow the membrane (and compositions groovedtherein) to follow the contour of the substrate topography. Such anintimate contact allows, in turn, the use of a composition (e.g., ametallic paste) with relatively low amounts of adhesive. As adhesivesneed to be eliminated for the electrically conductive particles in thecomposition to sinter and yield conductive lines, shapes consisting ofthe composition with lower amounts of adhesives are expected to undergoless and/or more predictable deformation/shape size reduction uponburning out of the adhesive, than were the composition to have higheramounts of adhesives. In methods in which, in contrast to the presentteachings, intimate contact cannot be obtained, the transfer, if any, istypically enabled by the inclusion of a layer of adhesive of relativelysignificant size (e.g., the adhesive having a thickness of 20% or moreof the height of the line, or 25% or more, or 30% or more). Rapidelimination of such an adhesive material layer may lead to unpredictableand/or uneven contacting between the base of the line and the substrate.Insufficient adhesion of a conductive line to a substrate may result ina relatively high contact resistance of the line over a unit of lengthof 1 cm. As the present invention allows satisfactory contact, in someembodiments, the normalized contact resistance of a conductive line(e.g., a grid line) prepared and transferred according to the presentteachings may be 0.3 Ω.cm or less, 0.25 Ω.cm or less, 0.2 Ω.cm or less,0.15 Ω.cm or less, 0.1 Ω.cm or less, or 0.08 Ω.cm or less (e.g.,assuming a sheet resistance of the substrate between 50 and 110 Ω/□, andconductive lines having a width of at least 15 μm and a height of atleast 8 μm).

A second series of parameters demonstrating the uniformity of aconductive line prepared and transferred according to the presentteachings relates to the profile of a cross-section section of the lineand the stability of its dimensions along the same line. Taking forcomparison lines applied by screen printing, a dominant metallizationtechnique for high-throughput production of solar cells, such lineswould display along their longitudinal axis a periodical fluctuation atleast in height of the lines. The valleys along the line would roughlycorrespond to the wires of the mesh used for screen printing, relativelylimiting the amount of composition able to flow to the substrate. Thepeaks along the line would roughly correspond to the apertures of themesh, allowing a higher flow of composition. The three-dimensionalparameters of a line (e.g., the cross-sectional dimensions of aconductive line, such as the line width, height, and/or cross-sectionarea) can be determined using a profilometer.

In the present example, two types of solar cells were compared, one typebeing conventionally prepared by screen-printing, the other beingprepared according to the present teachings. Within each type of solarcells, the conductive lines were intended to be identical to oneanother. Ten conductive lines of a commercially available screen-printedsolar cell and fifteen conductive lines (e.g., grid lines) in accordancewith the present invention were analysed using a true colour 3Dnon-contact optical profilometer (Zeta-20, Zeta Instruments, USA),including the “Solar Finger Analytics” image analysis software. Theprofilometer calculated the width and the height of the line on tenspaced cross-sections of each line, the cross-sections being randomlyselected at a distance of about 25-30 micrometres from one another. Foreach line, the average width and height of the line were calculated, soas the standard deviation of the width and height measurements,respectively.

Then the average measurements (AVE) of all lines prepared according to asame method were calculated.

Taking for illustration the width of the profiles of grid lines, eachwidth of each line shall be denominated W_(a)L_(b), wherein a is aninteger corresponding to a cross-section sequential number (e.g., 1, 2,3, etc. up to a final a value) and having a final a value of 10 or more(i.e., at least ten cross-sections per line), and wherein b is aninteger corresponding to a line sequential number (e.g., 1, 2, 3, etc.up to a final b value) and having a final b value of 5 or more (i.e., atleast five lines). In a first step, the mean width of each line iscalculated by summing up all values from W₁L_(b) to W_(a)L_(b) for everymeasured width of line b and by dividing the sum by the number ofcross-sections measured in said line (i.e. dividing by a). The meanwidth for line b so calculated can be represented by MeanW_(b) andsimilarly the standard deviation between all measured W₁L_(b) toW_(a)L_(b) can be represented by SD-W_(b). Such calculations arerepeated for each line until values are assigned to all MeanW₁ toMeanW_(b) and SD-W₁ to SD-W_(b). Then the arithmetical average of themean width and standard deviation can be calculated for all sampledlines, providing final values termed AVE-MeanW and AVE-SD-W. AVE-MeanHand AVE-SD-H, AVE-MeanS and AVE-SD-S, can be similarly calculated forany measured parameters, H representing here the height of the sampledcross-section profiles and S representing here the surface area of thesampled cross-section profiles. The variability is assessed as thepercentage value of AVE-SD divided by AVE-Mean (e.g., WidthVariability=100×(AVE-SD-W/AVE-MeanW).

All results and calculations are reported in Table 1 below, themeasurements being provided in micrometres.

TABLE 1 Screen-Printed Present Invention Line Mean SD Mean SD Mean SDMean SD # Height Height Width Width Height Height Width Width  1 18.312.129 64.53 6.231 15.50 0.466 47.58 1.764  2 15.11 2.406 55.12 5.25016.18 0.565 51.06 1.963  3 16.91 1.494 55.26 5.405 13.96 0.723 45.211.459  4 17.05 2.311 56.22 4.153 14.99 0.524 42.39 2.188  5 17.07 2.35957.63 6.567 14.37 0.494 46.29 1.864  6 17.31 2.064 61.66 6.236 14.200.550 47.14 3.617  7 12.13 1.828 62.40 5.375 12.21 0.448 38.60 1.271  817.10 1.878 60.20 7.017 12.20 0.316 42.70 2.138  9 16.77 2.203 56.885.377 12.28 0.542 39.25 1.575 10 16.89 2.325 58.71 7.131 14.89 0.57945.84 1.786 11 — — — — 15.41 0.610 48.91 2.214 12 — — — — 14.59 0.58146.45 2.809 13 — — — — 14.19 0.354 46.34 0.795 14 — — — — 14.23 0.78447.31 1.662 15 — — — — 15.56 0.636 44.96 2.608 AVE 16.47 2.100 58.865.874 14.32 0.545 45.34 1.981

As can be seen from the above-table, some fluctuations were observed inthe dimensions of the cross sections monitored by the profilometer. Itis noted that if there are errors in the monitored dimensions (e.g., asa result of a recurrent error in detection and/or analysis), it isexpected that the errors would be similar across the various monitoreddimensions, and therefore it is expected that the standard deviation andvariability values would be about equal to the values presented in Table1 and/or discussed below. For instance, automatically measured widthsmay be larger than widths manually measured by a trained operator ableto eliminate “noise” measurements resulting from the limitations of themeasuring instrument.

Lines prepared by screen-printing, once fired, displayed an averageheight for the ten lines that were sampled of about 16.47 μm with astandard deviation of about 2.100 μm, the average standard deviationhence representing approximately 12.9% of the average height. In otherwords, the screen-printed conductive lines have a height variability ofabout 12.9%. The average width of the screen-printed lines was about58.86 μm with a standard deviation of about 5.874 μm, the averagestandard deviation hence representing approximately 10.0% of the averagewidth. In other words, the screen-printed conductive lines have a widthvariability of about 10.0%. Roughly put, the standard deviations ofthese two dimensions for screen-printed lines were each at least 10% ofthe measured values (the variability in each dimension being of at least10%).

In contrast, lines prepared according to the present invention, oncefired, displayed an average height for the fifteen lines that weresampled of about 14.32 μm with a standard deviation of only about 0.545μm, the average standard deviation hence representing approximatelymerely 3.8% of the average height. In other words, the conductive linesprepared according to the present teachings have a height variability ofabout 3.8%. The average width of the lines prepared according to theinvention was of about 45.34 μm with a standard deviation of about 1.981μm, the average standard deviation hence representing approximately just4.4% of the average width. In other words, the conductive lines preparedaccording to the present teachings have a width variability of about4.4%. Roughly put, the standard deviations of these two dimensions forlines of the invention were each below 5% of the measured values (thevariability in each dimension achieved according to the presentinvention being of less than 5%). This at least two-fold lowervariability in the dimensions measured for lines of the invention ascompared to lines conventionally prepared by screen-printing is believedto be highly significant. While illustrated by the width and the heightof a cross-section of a conductive line, this low variability issimilarly expected for the area of the cross-section; the angle formedbetween the base of the cross-section and each of the rising slopesculminating in the apex of the cross-section; any calculations derivedfrom the dimensions and/or shape (including angles) of the cross-section(e.g., an averaged slope, corresponding to the mean of tan⁻¹ of theprofile slope (=derivative) of each angle at each sub-segment along thetapering face); etc.

Defining the variability of a measure as the percentage of the measuredstandard deviation with respect to the dimension being measured for atleast ten cross-sections per line along at least five conductive lines,the measurements being randomly spaced along each line, then in someembodiments of the present invention the width variability (variabilityin lines' width among lines designed to be substantially identical) is5% or less, 4% or less, or 3% or less, of the average of all measuredwidths. In some embodiments, the height variability (variability inlines' height among lines designed to be substantially identical) is 5%or less, 4% or less, or 3% or less, of the average of all measuredheights. In some embodiments, the cross-section area variability(variability in lines' cross-section area among lines designed to besubstantially identical) is 5% or less, 4% or less, or 3% or less, ofthe average of all measured cross section areas.

In some embodiments, the variability in angles' size at the base (leftside or right side) or apex of the cross-sections (or at any point alongthe profile of a conductive line wherein adjacent segments of theprofile are intended to form a non-planar angle) is expected to be 5% orless, 4% or less, or 3% or less, of the average of all measuredrespective angles. In some embodiments, the variability in averagedslope of the cross-sections' left tapering face is expected to be 5% orless, 4% or less, or 3% or less, of the average of all measured averagedslopes for said tapering face of the cross sections. In someembodiments, the variability in averaged slope of the cross-sections'right tapering face is expected to be 5% or less, 4% or less, or 3% orless, of the average of all measured averaged slopes for said taperingface of the cross sections. All afore-said variabilities are measuredand calculated for electrical conductors or conductive lines designed tobe substantially identical. Any of the above-detailed measurement (e.g.,cross-section width, height, area, angles, slopes, etc.) which can bemade on a cross-section of an electrical conductor or conductive linecan also be referred to as a cross-sectional dimension.

In some embodiments, the conductive lines (e.g., grid lines) prepared inaccordance with the present invention and their cross-section dimensionsdisplay low variability (of 5% or less) in more than one parameter asabove-described. For instance, a) both lines' width, and lines' heightmay display a low variability as above described; b) lines' width,lines' height and cross-section areas may display a low variability asabove described; c) lines' width, lines' height, cross-section areas andat least one angle of the cross-sections may display a low variabilityas above described; d) lines' width, lines' height, cross-section areas,and all angles of the cross-sections may display a low variability asabove described; and e) lines' width, lines' height, cross-sectionareas, all angles of the cross-sections and the averaged slopes of thetapering faces may display a low variability as above described.

In some embodiments, conductive lines prepared in accordance with thepresent invention, and their cross-sections, display low variability (of5% or less) in one or more parameter as above-described, and theconductive lines further display a low normalized contact resistance of0.3 Ω.cm or less, as previously detailed.

In the description and claims of the present disclosure, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements, steps or parts of thesubject or subjects of the verb.

As used herein, the singular form “a”, “an” and “the” include pluralreferences and mean “at least one” or “one or more” unless the contextclearly dictates otherwise.

Positional or motional terms such as “upper”, “lower”, “right” “left”,“bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”,“thigh”, “vertical”, “horizontal”, “backward”, “forward”, “upstream” and“downstream”, as well as grammatical variations thereof, may be usedherein for exemplary purposes only, to illustrate the relativepositioning, placement or displacement of certain components, toindicate a first and a second component in present illustrations or todo both. Such terms do not necessarily indicate that, for example, a“bottom” component is below a “top” component, as such directions,components or both may be flipped, rotated, moved in space, placed in adiagonal orientation or position, placed horizontally or vertically, orsimilarly modified.

Unless otherwise stated, the use of the expression “and/or” between thelast two members of a list of options for selection indicates that aselection of one or more of the listed options is appropriate and may bemade.

Unless otherwise stated, when the outer bounds of a range with respectto a feature of an embodiment of the present technology are noted in thedisclosure, it should be understood that in the embodiment, the possiblevalues of the feature may include the noted outer bounds as well asvalues in between the noted outer bounds. In the disclosure, unlessotherwise stated, adjectives such as “substantially”, “approximately”and “about” that modify a condition or relationship characteristic of afeature or features of an embodiment of the present technology, are tobe understood to mean that the condition or characteristic is defined towithin tolerances that are acceptable for operation of the embodimentfor an application for which it is intended, or within variationsexpected from the measurement being performed and/or from the measuringinstrument being used. Furthermore, unless otherwise stated, the terms(e.g., numbers) used in this disclosure, even without such adjectives,should be construed as having tolerances which may depart from theprecise meaning of the relevant term but would enable the invention orthe relevant portion thereof to operate and function as described, andas understood by a person skilled in the art.

Certain marks referenced herein may be common law or registeredtrademarks of third parties. Use of these marks is by way of example andshall not be construed as descriptive or limit the scope of thisdisclosure to material associated only with such marks.

While this disclosure has been described in terms of certain embodimentsand generally associated methods, alterations and permutations of theembodiments and methods will be apparent to those skilled in the art.The present disclosure is to be understood as not limited by thespecific examples described herein.

1. A method of applying a pattern of electrical conductors to asubstrate formed by a surface of a solar cell, which method comprises:a) providing a flexible membrane, wherein a first surface of themembrane has a pattern of grooves formed therein, the patterncorresponding at least partially to a desired pattern of electricalconductors to be applied to the substrate; b) loading into the groovesof the first surface of the membrane a composition that includes, ascomposition components, electrically conductive particles and anadhesive, said loading being performed in one or more filling cycle(s)such that on completion of loading the composition substantially fillsthe grooves, level with the first surface of the membrane, and parts ofthe first surface between the grooves are substantially devoid of thecomposition; c) contacting the membrane with the substrate, with thefirst surface of the membrane facing the substrate; d) applying pressureto the membrane to cause the composition loaded into the grooves in thefirst surface of the membrane to adhere to the substrate; e) separatingthe membrane from the substrate to transfer the composition from thegrooves in the first surface of the membrane to the substrate; and f)applying sufficient energy to sinter the electrically conductiveparticles and to render electrically conductive the pattern ofcomposition transferred to the substrate from the grooves.
 2. The methodof claim 1, wherein the composition loaded into the grooves comprises,as an additional composition component, a liquid carrier to form a wetcomposition, and wherein each filling cycle includes the steps of: (i)applying an excess of the wet composition to the whole of the firstsurface of the membrane; (ii) removing excess wet composition from thefirst surface to leave the wet composition substantially only within thegrooves in the first surface of the membrane; and (iii) substantiallydrying the wet composition within the grooves by removing the liquidcarrier to leave dried composition.
 3. The method of claim 2, whereinthe relative proportions of the components of the composition appliedduring different filling cycles differ from one another.
 4. The methodof claim 1, wherein a release coating is applied to the grooves of themembrane and dried thereon, prior to loading of the composition into thegrooves.
 5. The method of claim 1, further comprising, prior to step b),the steps of: A. applying a light modifying coating to the first surfaceof the membrane, B. wiping the first surface to leave the coatingsubstantially only within the grooves, and C. subsequently drying thelight modifying coating to leave the dried light modifying coating onlyon the walls of the grooves.
 6. The method of claim 1, wherein prior tocontacting said membrane with said substrate according to step c), anadhesive coating is applied to at least one of the first surface of themembrane and the substrate so as to coat any composition present in thegrooves, a dried thickness of said adhesive coating not exceeding 2 μm.7. The method of claim 1, wherein at least two grooves in the pattern,or two different segments of the same groove, have differentcross-sectional dimensions.
 8. The method of claim 1, wherein thesubstrate comprises a semiconductor wafer, and wherein at least one ofthe compositions loaded into the grooves includes a glass frit, as anadditional composition component, and wherein the substrate and thecomposition are heated, following step e), to cause the composition tofuse with the substrate.
 9. The method of claim 1, wherein the flexiblemembrane is selected from a preformed membrane of plastics polymer and acast plastics polymer.
 10. The method of claim 9, wherein the plasticspolymer is a thermoplastic polymer, selected from the group consistingof cyclic olefin copolymer (COC), polyethylene (PE), cast polypropylene(CPP), any other type of polypropylene (PP), thermoplastic polyurethane(TPU), and combinations thereof.
 11. The method of claim 1 wherein thefirst surface of the flexible membrane has a mean roughness Rz of 1 μmor less.
 12. The method of claim 1, wherein the particles ofelectrically conductive material are made of compounds selected from thegroup consisting of metals, alloys, organo-metals, conductive polymers,conductive polymers precursors, salts thereof and combinations thereof.13. The method of claim 1, wherein the adhesive includes at least one of(i) an organic binder, (ii) an organic adhesive that is a pressureand/or heat sensitive adhesive, and (iii) a glass frit.
 14. The methodof claim 1, wherein the step of providing a flexible membrane having apattern of grooves formed in a first surface thereof comprises advancinga continuous membrane between a die roller and a counter die, the dieroller having protruding rules complementary to the pattern of groovesto be formed on the membrane.
 15. The method of claim 1, wherein thestep of applying sufficient energy to sinter the electrically conductiveparticles comprises raising heating the composition at at least onetemperature within a range of 150° C. to 800° C. for a duration notexceeding two minutes.
 16. The method of claim 1, wherein the pattern ofgrooves includes longitudinal grooves forming contiguous or distinctstraight or curved lines, at least a portion of each line having atapering cross section profile selected from a triangular, trapezoidal,polygonal, semi-circular, or semi-elliptic profile, any of the profileshaving at least a base width W_(B) and a height h, the dimensionlessaspect ratio ASP between the height and the base width being within arange of 5:1 to 0.7:1.
 17. The method of claim 16, wherein theelectrically conductive pattern of electrical conductors includes gridlines for collecting current generated by the solar cell, and wherein asum of the lengths of any segments of a grid line cross-section widththat are inclined to the plane of the substrate at an angle of less than45° does not exceed 15 μm.
 18. The method of claim 1, wherein theelectrically conductive pattern of electrical conductors includes gridlines for collecting current generated by the solar cell, wherein when,for at least five grid lines, at least ten width measurements are madeat spaced points along each grid line and a mean width value and astandard deviation is calculated for each grid line, the average of thestandard deviations calculated for the at least five grid lines is nomore than 5% of the average of the mean width values calculated for theat least five grid lines.
 19. The method of claim 1, wherein theelectrically conductive pattern of electrical conductors includes gridlines for collecting current generated by the solar cell, said gridlines having a normalized contact resistance of at most 0.3 Ω.cm whenthe substrate has a sheet resistance of at least 50 Ω/□ (i.e. 50 Ω/sq)and at most 110 Ω/□ and when the grid lines have a width of at least 15μm and a height of at least 8 μm; or the grid lines have a normalizedcontact resistance of at most 0.6 Ω.cm when the substrate has a sheetresistance of at least 111 Ω/□ and at most 140 Ω/□, and when the gridlines have a width of at least 15 μm and a height of at least 8 μm. 20.The method of claim 1, further comprising following step (d), coolingthe membrane and substrate down to a predetermined temperature, whereinin step (d) the pressure applied to the membrane also causes themembrane to retain contact with the substrate until after the membraneand substrate have cooled down to the predetermined temperature,regardless of any amount of adhesive between the first surface of themembrane and the substrate.
 21. A flexible membrane suitable forapplying a pattern of electrical conductors to a substrate formed by asurface of a solar cell, the membrane having a first surface thatincludes a pattern of grooves, the grooves being loaded so as to besubstantially level with the surface of the membrane with a compositionthat includes electrically conductive particles and an adhesive, thecomposition being adapted to become electrically conductive uponsintering by application of energy thereto, and ungrooved parts of thefirst surface being substantially devoid of the composition; themembrane being such that upon pressing the membrane and the substrateagainst one another, the composition in the grooves adheres morestrongly to the substrate than to the membrane, and such that subsequentseparation of the membrane from the substrate results in the compositionremaining on the substrate in a pattern mirroring that of the grooves.22. A solar cell consisting of a substrate and an electricallyconductive pattern comprising a pattern of electrical conductorsincluding grid lines for collecting current generated by the solar cell,wherein the pattern of electrical conductors is applied to the substrateby a method comprising: a) providing a flexible membrane, wherein afirst surface of the membrane has a pattern of grooves formed therein,the pattern corresponding at least partially to a desired pattern ofelectrical conductors to be applied to the substrate; b) loading intothe grooves of the first surface of the membrane a composition thatincludes, as composition components, electrically conductive particlesand an adhesive, said loading being performed in one or more fillingcycle(s) such that on completion of loading the compositionsubstantially fills the grooves, level with the first surface of themembrane, and parts of the first surface between the grooves aresubstantially devoid of the composition; c) contacting the membrane withthe substrate, with the first surface of the membrane facing thesubstrate; d) applying pressure to the membrane to cause the compositionloaded into the grooves in the first surface of the membrane to adhereto the substrate; e) separating the membrane from the substrate totransfer the composition from the grooves in the first surface of themembrane to the substrate; and f) applying sufficient energy to sinterthe electrically conductive particles and to render electricallyconductive the pattern of composition transferred to the substrate fromthe grooves.
 23. A solar cell having an electrically conductive patterncomprising a pattern of electrical conductors applied on a lightharvesting side of the solar cell, the electrical conductors includinggrid lines for collecting current generated by the solar cell, whereinat least part of the electrical conductors fulfils the followingstructural features: a) the electrical conductors have a cross-sectionprofile with a base of a width W_(B) contacting the light harvestingside of the solar cell, an orthogonal distance h between the base and atop point of the profile (also termed the height of the profile), and anaspect ratio ASP between the height of the profile and the base width(ASP=h/W_(B)), wherein: i. W_(B) is at most 50 μm, ii. h is at most 50μm; and iii. ASP is at least 0.7:1 and at most 5:1; b) wherein when, forat least five grid lines, at least ten width measurements are made atspaced points along each grid line and a mean width value and a standarddeviation is calculated for each grid line, the average of the standarddeviations calculated for the at least five grid lines is no more than5% of the average of the mean width values calculated for the at leastfive grid lines; c) wherein when, for at least five grid lines, at leastten height measurements are made at spaced points along each grid lineand a mean height value and a standard deviation is calculated for eachgrid line, the average of the standard deviations calculated for the atleast five grid lines is no more than 5% of the average of the meanheight values calculated for the at least five grid lines; and d) thegrid lines have a normalized contact resistance of at most 0.3 Ω.cm whena sheet resistance of the light harvesting side is at least 50 Ω/□ andat most 110 Ω/□ and when the grid lines have a width of at least 15 μmand a height of at least 8 μm, or the grid lines have a normalizedcontact resistance of at most 0.6 Ω.cm when a sheet resistance of thelight harvesting side is at least 111 Ω/□ and at most 140 Ω/□, and whenthe grid lines have a width of at least 15 μm and a height of at least 8μm.
 24. The solar cell of claim 23, wherein the grid lines fulfilfeatures a) to d).
 25. The solar cell of claim 23, wherein at least partof the electrical conductors additionally fulfils one or more of thefollowing features: a. the electrical conductors consist of one or moresintered electrically conductive materials; b. a cross-section slice ofthe electrical conductors perpendicular to the light harvesting side ofthe solar cell comprises two or more layers of different sinteredmaterials, the layers optionally following the profile of thecross-section of the conductors, and symmetric on opposite sides of anaxis traversing the layers; c. the grid lines have an external surfacewith light reflective and low light diffusivity properties; d. across-section profile of the electrical conductors has at least twosides rising from a base of the profile, wherein an average of a slopealong half of the width of the basis, as assessed by the tangent valueof the angles formed between at least one of the two sides and the baseof the profile, is at least 1.4 and at most 10; e. a sum of the lengthsof any segments of a grid line cross-section that are inclined to theplane of the substrate at an angle of less than 45° does not exceed 15μm; and f. wherein when, for at least five grid lines, at least tencross-section area measurements are made at spaced points along eachgrid line and a mean cross-section area value and a standard deviationis calculated for each grid line, the average of the standard deviationscalculated for the at least five grid lines is no more than 5% of theaverage of the mean cross-section area values calculated for the atleast five grid lines.